diff --git a/db/migrations/2018_10_01_000000_create_users_tables.php b/db/migrations/2018_10_01_000000_create_users_tables.php
index 1a847465..d2fee72e 100644
--- a/db/migrations/2018_10_01_000000_create_users_tables.php
+++ b/db/migrations/2018_10_01_000000_create_users_tables.php
@@ -93,7 +93,7 @@ class CreateUsersTables extends Migration
                     'password'      => $data->Passwort,
                     'email'         => $data->email,
                     'api_key'       => $data->api_key,
-                    'last_login_at' => Carbon::createFromTimestamp($data->lastLogIn),
+                    'last_login_at' => $data->lastLogIn ? Carbon::createFromTimestamp($data->lastLogIn) : null,
                 ]);
                 $user->setAttribute('id', $data->UID);
                 if (!in_array($data->CreateDate, $emptyDates)) {
diff --git a/docker/Dockerfile b/docker/Dockerfile
index c1c334ef..b6046ecc 100644
--- a/docker/Dockerfile
+++ b/docker/Dockerfile
@@ -28,7 +28,7 @@ COPY --from=composer /app/composer.lock /app/
 RUN find /app/storage/ -type f -not -name VERSION -exec rm {} \;
 
 # Build the PHP container
-FROM php:8.0-fpm-alpine
+FROM php:8-fpm-alpine
 WORKDIR /var/www
 RUN apk add --no-cache icu-dev && \
     docker-php-ext-install intl pdo_mysql
diff --git a/docker/dev/Dockerfile b/docker/dev/Dockerfile
index fc5e489a..cc89b9f4 100644
--- a/docker/dev/Dockerfile
+++ b/docker/dev/Dockerfile
@@ -1,5 +1,5 @@
 # Engelsystem PHP FPM development image including Xdebug
-FROM php:8.0-fpm-alpine AS es_php_fpm
+FROM php:8-fpm-alpine AS es_php_fpm
 WORKDIR /var/www
 RUN apk add --no-cache icu-dev $PHPIZE_DEPS && \
     pecl install pcov xdebug && \
diff --git a/includes/sys_form.php b/includes/sys_form.php
index 9655935c..f96da233 100644
--- a/includes/sys_form.php
+++ b/includes/sys_form.php
@@ -11,7 +11,7 @@ use Carbon\Carbon;
  */
 function form_hidden($name, $value)
 {
-    return '<input type="hidden" name="' . $name . '" value="' . htmlspecialchars($value) . '" />';
+    return '<input type="hidden" name="' . $name . '" value="' . htmlspecialchars((string)$value) . '" />';
 }
 
 /**
@@ -24,7 +24,7 @@ function form_hidden($name, $value)
  */
 function form_spinner($name, $label, $value)
 {
-    $value = htmlspecialchars($value);
+    $value = htmlspecialchars((string)$value);
 
     return form_element($label, '
         <div class="input-group">
@@ -69,7 +69,7 @@ function form_date($name, $label, $value, $start_date = '', $end_date = '')
 
     return form_element($label, '
     <div class="input-group date" id="' . $dom_id . '" data-min-date="' . $start_date . '" data-max-date="' . $end_date . '" data-target-input="nearest">
-        <input type="date" placeholder="YYYY-MM-DD" name="' . $name . '" class="form-control" value="' . htmlspecialchars($value) . '" autocomplete="off">'
+        <input type="date" placeholder="YYYY-MM-DD" name="' . $name . '" class="form-control" value="' . htmlspecialchars((string)$value) . '" autocomplete="off">'
         . '<span class="input-group-text">' . icon('grid-3x3-gap-fill') . '</span>
     </div>
     ', $dom_id);
@@ -178,7 +178,7 @@ function form_checkbox($name, $label, $selected, $value = 'checked', $html_id =
     }
 
     return '<div class="checkbox"><label>'
-        . '<input type="checkbox" id="' . $html_id . '" name="' . $name . '" value="' . htmlspecialchars($value) . '" '
+        . '<input type="checkbox" id="' . $html_id . '" name="' . $name . '" value="' . htmlspecialchars((string)$value) . '" '
         . ($selected ? ' checked="checked"' : '') . ' /> '
         . $label
         . '</label></div>';
@@ -196,7 +196,7 @@ function form_checkbox($name, $label, $selected, $value = 'checked', $html_id =
 function form_radio($name, $label, $selected, $value)
 {
     return '<div class="radio">'
-        . '<label><input type="radio" id="' . $name . '" name="' . $name . '" value="' . htmlspecialchars($value) . '" '
+        . '<label><input type="radio" id="' . $name . '" name="' . $name . '" value="' . htmlspecialchars((string)$value) . '" '
         . ($selected ? ' checked="checked"' : '') . ' /> '
         . $label
         . '</label></div>';
@@ -268,7 +268,7 @@ function form_text($name, $label, $value, $disabled = false, $maxlength = null,
     return form_element(
         $label,
         '<input class="form-control" id="form_' . $name . '" type="text" name="' . $name
-        . '" value="' . htmlspecialchars($value) . '"' . $maxlength . $disabled . $autocomplete . '/>',
+        . '" value="' . htmlspecialchars((string)$value) . '"' . $maxlength . $disabled . $autocomplete . '/>',
         'form_' . $name,
         $class
     );
@@ -288,7 +288,7 @@ function form_text_placeholder($name, $placeholder, $value, $disabled = false)
     $disabled = $disabled ? ' disabled="disabled"' : '';
     return form_element('',
         '<input class="form-control" id="form_' . $name . '" type="text" name="' . $name
-        . '" value="' . htmlspecialchars($value) . '" placeholder="' . $placeholder
+        . '" value="' . htmlspecialchars((string)$value) . '" placeholder="' . $placeholder
         . '" ' . $disabled . '/>'
     );
 }
@@ -313,7 +313,7 @@ function form_email($name, $label, $value, $disabled = false, $autocomplete = nu
     return form_element(
         $label,
         '<input class="form-control" id="form_' . $name . '" type="email" name="' . $name . '" value="'
-        . htmlspecialchars($value) . '" ' . $disabled . $autocomplete . $maxlength . '/>',
+        . htmlspecialchars((string)$value) . '" ' . $disabled . $autocomplete . $maxlength . '/>',
         'form_' . $name
     );
 }
@@ -391,7 +391,7 @@ function form_textarea($name, $label, $value, $disabled = false)
     return form_element(
         $label,
         '<textarea rows="5" class="form-control" id="form_' . $name . '" name="'
-        . $name . '" ' . $disabled . '>' . htmlspecialchars($value) . '</textarea>',
+        . $name . '" ' . $disabled . '>' . htmlspecialchars((string)$value) . '</textarea>',
         'form_' . $name
     );
 }
diff --git a/includes/view/AngelTypes_view.php b/includes/view/AngelTypes_view.php
index ef6d1597..692bb590 100644
--- a/includes/view/AngelTypes_view.php
+++ b/includes/view/AngelTypes_view.php
@@ -403,7 +403,7 @@ function AngelType_view_info(
     $info[] = '<h3>' . __('Description') . '</h3>';
     $parsedown = new Parsedown();
     if ($angeltype['description'] != '') {
-        $info[] = $parsedown->parse($angeltype['description']);
+        $info[] = $parsedown->parse((string)$angeltype['description']);
     }
 
     list($supporters, $members_confirmed, $members_unconfirmed) = AngelType_view_members(
@@ -561,7 +561,7 @@ function AngelTypes_about_view_angeltype($angeltype)
         );
     }
     if ($angeltype['description'] != '') {
-        $html .= $parsedown->parse($angeltype['description']);
+        $html .= $parsedown->parse((string)$angeltype['description']);
     }
     $html .= '<hr />';
 
diff --git a/includes/view/Rooms_view.php b/includes/view/Rooms_view.php
index a2430729..10d0f723 100644
--- a/includes/view/Rooms_view.php
+++ b/includes/view/Rooms_view.php
@@ -24,7 +24,7 @@ function Room_view(Room $room, ShiftsFilterRenderer $shiftsFilterRenderer, Shift
     if ($room->description) {
         $description = '<h3>' . __('Description') . '</h3>';
         $parsedown = new Parsedown();
-        $description .= $parsedown->parse($room->description);
+        $description .= $parsedown->parse((string)$room->description);
     }
 
     $tabs = [];
diff --git a/includes/view/ShiftTypes_view.php b/includes/view/ShiftTypes_view.php
index 5a4b1322..d8e0510c 100644
--- a/includes/view/ShiftTypes_view.php
+++ b/includes/view/ShiftTypes_view.php
@@ -98,7 +98,7 @@ function ShiftType_view($shifttype, $angeltype)
             )
         ]),
         heading(__('Description'), 2),
-        $parsedown->parse($shifttype['description'])
+        $parsedown->parse((string)$shifttype['description'])
     ], true);
 }
 
diff --git a/includes/view/Shifts_view.php b/includes/view/Shifts_view.php
index e4a2df6c..859c723e 100644
--- a/includes/view/Shifts_view.php
+++ b/includes/view/Shifts_view.php
@@ -176,8 +176,8 @@ function Shift_view($shift, $shifttype, Room $room, $angeltypes_source, ShiftSig
         ]),
         div('col-sm-6', [
             '<h2>' . __('Description') . '</h2>',
-            $parsedown->parse($shifttype['description']),
-            $parsedown->parse($shift['description']),
+            $parsedown->parse((string)$shifttype['description']),
+            $parsedown->parse((string)$shift['description']),
         ])
     ]);
 
diff --git a/src/Helpers/ConfigureEnvironmentServiceProvider.php b/src/Helpers/ConfigureEnvironmentServiceProvider.php
index 1660de1e..6722ce55 100644
--- a/src/Helpers/ConfigureEnvironmentServiceProvider.php
+++ b/src/Helpers/ConfigureEnvironmentServiceProvider.php
@@ -32,7 +32,7 @@ class ConfigureEnvironmentServiceProvider extends ServiceProvider
      */
     protected function setTimeZone(CarbonTimeZone $timeZone)
     {
-        ini_set('date.timezone', $timeZone);
+        ini_set('date.timezone', (string)$timeZone);
         date_default_timezone_set($timeZone);
     }
 
diff --git a/src/Http/SessionHandlers/AbstractHandler.php b/src/Http/SessionHandlers/AbstractHandler.php
index 135d0d43..651b8b61 100644
--- a/src/Http/SessionHandlers/AbstractHandler.php
+++ b/src/Http/SessionHandlers/AbstractHandler.php
@@ -43,7 +43,8 @@ abstract class AbstractHandler implements SessionHandlerInterface
      * @param int $maxLifetime
      * @return bool
      */
-    public function gc($maxLifetime): bool
+    #[\ReturnTypeWillChange]
+    public function gc($maxLifetime)
     {
         return true;
     }
diff --git a/src/Http/SessionHandlers/DatabaseHandler.php b/src/Http/SessionHandlers/DatabaseHandler.php
index 8df70287..dafdaa89 100644
--- a/src/Http/SessionHandlers/DatabaseHandler.php
+++ b/src/Http/SessionHandlers/DatabaseHandler.php
@@ -74,7 +74,7 @@ class DatabaseHandler extends AbstractHandler
     /**
      * {@inheritdoc}
      */
-    public function gc($maxLifetime): bool
+    public function gc($maxLifetime)
     {
         $timestamp = $this->getCurrentTimestamp(-$maxLifetime);
 
diff --git a/src/Logger/Logger.php b/src/Logger/Logger.php
index e3d3cacd..ee4a9854 100644
--- a/src/Logger/Logger.php
+++ b/src/Logger/Logger.php
@@ -72,7 +72,7 @@ class Logger extends AbstractLogger
             }
 
             // replace the values of the message
-            $message = str_replace('{' . $key . '}', $val, $message);
+            $message = str_replace('{' . $key . '}', (string)$val, $message);
         }
 
         return $message;
diff --git a/src/Middleware/ErrorHandler.php b/src/Middleware/ErrorHandler.php
index a3bd6f7b..6caae3c9 100644
--- a/src/Middleware/ErrorHandler.php
+++ b/src/Middleware/ErrorHandler.php
@@ -77,7 +77,7 @@ class ErrorHandler implements MiddlewareInterface
         $statusCode = $response->getStatusCode();
         $contentType = $response->getHeader('content-type');
         $contentType = array_shift($contentType);
-        if (!$contentType && strpos($response->getBody(), '<html') !== false) {
+        if (!$contentType && strpos($response->getBody() ?? '', '<html') !== false) {
             $contentType = 'text/html';
         }
 
diff --git a/src/Middleware/SendResponseHandler.php b/src/Middleware/SendResponseHandler.php
index 34e70a87..66d196d7 100644
--- a/src/Middleware/SendResponseHandler.php
+++ b/src/Middleware/SendResponseHandler.php
@@ -64,6 +64,10 @@ class SendResponseHandler implements MiddlewareInterface
      */
     protected function sendHeader($content, $replace = true, $code = null)
     {
-        header($content, $replace, $code);
+        if (is_null($code)) {
+            header($content, $replace);
+        } else {
+            header($content, $replace, $code);
+        }
     }
 }
diff --git a/src/Models/EventConfig.php b/src/Models/EventConfig.php
index f31948d2..965d2d32 100644
--- a/src/Models/EventConfig.php
+++ b/src/Models/EventConfig.php
@@ -50,7 +50,7 @@ class EventConfig extends BaseModel
      */
     public function getValueAttribute($value)
     {
-        $value = $this->fromJson($value);
+        $value = $value ? $this->fromJson($value) : null;
 
         /** @see \Illuminate\Database\Eloquent\Concerns\HasAttributes::castAttribute */
         if (!empty($value)) {
diff --git a/tests/Unit/Models/EventConfigTest.php b/tests/Unit/Models/EventConfigTest.php
index 76b30061..ebc5e827 100644
--- a/tests/Unit/Models/EventConfigTest.php
+++ b/tests/Unit/Models/EventConfigTest.php
@@ -78,6 +78,10 @@ class EventConfigTest extends ModelTest
                 ->value
                 ->format('Y-m-d H:i')
         );
+        $this->assertEquals(
+            null,
+            ($this->getEventConfig())->getValueAttribute(null)
+        );
     }
 
     /**