[otrs] Slow QueueView and error message after upgrading OTRS Free from 5.0.24 to 6.0.1

Hello, just updated my OTRS instance and the upgrade process passed without errors. After the login the view for some queues is very slow (~8 seconds) while only 7 tickets are inside. The syslog is full of messages like this one -> Nov 23 17:38:04 ttsngdev OTRS-CGI-10[1372]: [Error][Kernel::System::Group::PermissionCheck][Line:945]: Need GroupName! Nov 23 17:38:04 ttsngdev OTRS-CGI-10[1372]: [Error][Kernel::System::Group::PermissionCheck][Line:945]: Need GroupName! I ran the SetPermissions script twice....but no change. Maybe someone have an idea? Thank you very much! Julius Albert