Unsure about version affected, the error happened in Area51, so that will tell the version.

I have the following situation:

1. I have a notification in my received notifications, it is active, so the number of active notifications shows as "1" . I open the notifications dropdown and mark it to be cleared. It clears, but when I reload the page, there it is again the same notification, active again.
2. Then I go to the UCP to manage notifications, get on the list of active notifications, select this one, and click on "Mark read", and I get a nice php error:
   {{[phpBB Debug] PHP Warning: in file [ROOT]/vendor/symfony/dependency-injection/ContainerBuilder.php on line 481: strtolower() expects parameter 1 to be string, array given

Fatal error: Uncaught exception 'Symfony\Component\DependencyInjection\Exception\InvalidArgumentException' with message 'The service definition "" does not exist.' in
/var/www/phpbb/area51.phpbb.com/area51-phpbb3/phpBB/vendor/symfony/dependency-injection/ContainerBuilder.php:882
Stack trace:
#0 /var/www/phpbb/area51.phpbb.com/area51-phpbb3/phpBB/vendor/symfony/dependency-injection/ContainerBuilder.php(492): Symfony\Component\DependencyInjection\ContainerBuilder->getDefinition(NULL)
#1 /var/www/phpbb/area51.phpbb.com/area51-phpbb3/phpBB/phpbb/notification/manager.php(858): Symfony\Component\DependencyInjection\ContainerBuilder->get(Array)
#2 /var/www/phpbb/area51.phpbb.com/area51-phpbb3/phpBB/phpbb/notification/manager.php(848): phpbb\notification\manager->load_object(Array)
#3 /var/www/phpbb/area51.phpbb.com/area51-phpbb3/phpBB/phpbb/notification/manager.php(226): phpbb\notification\manager->get_method_class(Array)
#4 /var/www/phpbb/area51.phpbb.com/area51-phpbb3/phpBB/includes/ucp/ucp_notifications.php(125): phpbb\notifi in /var/www/phpbb/area51.phpbb.com/area51-phpbb3/phpBB/vendor/symfony/dependency-injection/ContainerBuilder.php on line 882
}}
For context, the notification is a "Report closed" notification.