I tried to create an RBD via the UI and got a toasty message saying "nagios3 --verify config failed" (and got stuck in the RBD-add form).
Clicked on "back" and recognized that the RBD actually exists.
Ran nagios3 --verify-config /etc/nagios3/nagios.cfg and got this error:
Error: Contact group 'openattic_admins' specified in service 'CPU Time' for host '<hostname>' is not defined anywhere!This error also occurred for Disk stats checks.
The problem was gone after running "groupadd openattic_admins" - --verify-config had no failures after that