"Exception: Call to a member function getUID() on null" every 3-5 Minutes in Nextcloud log

Nextcloud version (eg, 20.0.5): 21.0.5
Operating system and version (eg, Ubuntu 20.04): Linux 4.15.0-159-generic x86_64 (no more information because it is a hosted environment)
Apache or nginx version (eg, Apache 2.4.25): Unknown (hosted environment)
PHP version (eg, 7.4): 7.4.23

The issue you are facing:
Our nextcloud log shows a new error every 3-5 Minutes. It is always the same error message but for different users.

Is this the first time you’ve seen this error? (Y/N): Y

Steps to replicate it:

Unknown

The output of your Nextcloud log in Admin > Logging:

{"reqId":"reqId","level":3,"time":"2021-10-31T07:46:33+00:00","remoteAddr":"0.0.0.0","user":"user@domain.com","app":"no app in context","method":"GET",
"url":"/ocs/v2.php/apps/notifications/api/v2/notifications",
"message":{"Exception":"Exception","Message":"Call to a member function getUID() on null","Code":0,
"Trace":[{"file":"/www/htdocs/w00b5f79/nextcloud20/lib/private/AppFramework/App.php","line":157,"function":"dispatch","class":"OC\\AppFramework\\Http\\Dispatcher","type":"->","args":[{"__class__":"OCA\\Notifications\\Controller\\EndpointController"},"listNotifications"]},
{"file":"/www/htdocs/w00b5f79/nextcloud20/lib/private/Route/Router.php","line":302,"function":"main","class":"OC\\AppFramework\\App","type":"::","args":["OCA\\Notifications\\Controller\\EndpointController","listNotifications",{"__class__":"OC\\AppFramework\\DependencyInjection\\DIContainer"},{"apiVersion":"v2","_route":"ocs.notifications.Endpoint.listNotifications"}]},
{"file":"/www/htdocs/w00b5f79/nextcloud20/ocs/v1.php","line":63,"function":"match","class":"OC\\Route\\Router","type":"->","args":["/ocsapp/apps/notifications/api/v2/notifications"]},
{"file":"/www/htdocs/w00b5f79/nextcloud20/ocs/v2.php","line":24,"args":["/www/htdocs/w00b5f79/nextcloud20/ocs/v1.php"],"function":"require_once"}],"File":"/www/htdocs/w00b5f79/nextcloud20/lib/private/AppFramework/Http/Dispatcher.php","Line":159,"Previous":{"Exception":"Error","Message":"Call to a member function getUID() on null","Code":0,
"Trace":[{"file":"/www/htdocs/w00b5f79/nextcloud20/apps/polls/lib/Notification/Notifier.php","line":122,"function":"getActor","class":"OCA\\Polls\\Notification\\Notifier","type":"->","args":[null]},
{"file":"/www/htdocs/w00b5f79/nextcloud20/lib/private/Notification/Manager.php","line":299,"function":"prepare","class":"OCA\\Polls\\Notification\\Notifier","type":"->","args":[{"__class__":"OC\\Notification\\Notification"},"de"]},
{"file":"/www/htdocs/w00b5f79/nextcloud20/apps/notifications/lib/Controller/EndpointController.php","line":106,"function":"prepare","class":"OC\\Notification\\Manager","type":"->","args":[{"__class__":"OC\\Notification\\Notification"},"de"]},
{"file":"/www/htdocs/w00b5f79/nextcloud20/lib/private/AppFramework/Http/Dispatcher.php","line":218,"function":"listNotifications","class":"OCA\\Notifications\\Controller\\EndpointController","type":"->","args":["v2"]},
{"file":"/www/htdocs/w00b5f79/nextcloud20/lib/private/AppFramework/Http/Dispatcher.php","line":127,"function":"executeController","class":"OC\\AppFramework\\Http\\Dispatcher","type":"->","args":[{"__class__":"OCA\\Notifications\\Controller\\EndpointController"},"listNotifications"]},
{"file":"/www/htdocs/w00b5f79/nextcloud20/lib/private/AppFramework/App.php","line":157,"function":"dispatch","class":"OC\\AppFramework\\Http\\Dispatcher","type":"->","args":[{"__class__":"OCA\\Notifications\\Controller\\EndpointController"},"listNotifications"]},
{"file":"/www/htdocs/w00b5f79/nextcloud20/lib/private/Route/Router.php","line":302,"function":"main","class":"OC\\AppFramework\\App","type":"::","args":["OCA\\Notifications\\Controller\\EndpointController","listNotifications",{"__class__":"OC\\AppFramework\\DependencyInjection\\DIContainer"},{"apiVersion":"v2","_route":"ocs.notifications.Endpoint.listNotifications"}]},
{"file":"/www/htdocs/w00b5f79/nextcloud20/ocs/v1.php","line":63,"function":"match","class":"OC\\Route\\Router","type":"->","args":["/ocsapp/apps/notifications/api/v2/notifications"]},
{"file":"/www/htdocs/w00b5f79/nextcloud20/ocs/v2.php","line":24,"args":["/www/htdocs/w00b5f79/nextcloud20/ocs/v1.php"],"function":"require_once"}],"File":"/www/htdocs/w00b5f79/nextcloud20/apps/polls/lib/Notification/Notifier.php","Line":96},"CustomMessage":"--"},"userAgent":"Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/95.0.4638.54 Safari/537.36","version":"21.0.5.1"}

The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):

<?php
$CONFIG = array (
  'instanceid' => 'instanceid',
  'passwordsalt' => 'passwordsalt',
  'secret' => 'secret',
  'trusted_domains' => 
  array (
    0 => 'cloud.domain.com',
    1 => 'www.cloud.domain.com',
  ),
  'datadirectory' => '/www/htdocs/nextcloud/data',
  'dbtype' => 'mysql',
  'version' => '21.0.5.1',
  'overwrite.cli.url' => 'https://cloud.domain.com/',
  'dbname' => 'dbname',
  'dbhost' => 'localhost',
  'dbport' => '',
  'dbtableprefix' => 'nc_',
  'mysql.utf8mb4' => true,
  'dbuser' => 'dbuser',
  'dbpassword' => 'dbpassword',
  'installed' => true,
  'tempdirectory' => '/www/htdocs/nextcloud/tmp/',
  'mail_smtpmode' => 'sendmail',
  'mail_sendmailmode' => 'pipe',
  'mail_from_address' => 'mail',
  'mail_domain' => 'domain.com',
  'maintenance' => false,
  'theme' => '',
  'loglevel' => 2,
  'htaccess.RewriteBase' => '/',
  'simpleSignUpLink.shown' => false,
  'default_language' => 'de',
  'default_locale' => 'de_DE',
  'skeletondirectory' => '',
  'trashbin_retention_obligation' => 'auto, 30',
  'updater.release.channel' => 'stable',
  'app_install_overwrite' => 
  array (
    0 => 'files_3d',
  ),
  'updater.secret' => 'updater.secret',
);

The output of your Apache/nginx/system log in /var/log/____:

Not accessible (hosted environment)