It actually seems not connected to CBsubs. It seems to be a Joomla bug. Sorry for posting it as a CBsubs bug here.
The reason I noticed it however is, that I changed a lot of joomla access levels to registered, in order to make things simpler and arrange actual access for special users with the CBsubs restriction, getting invitations up for upgrading plans for entering.
So be aware that when doing so, you don´t grant access to items for which the corresponding user level is insufficient to show the content, because than this error might be te result.
So: a menu item link to an article category which is of access level special clicked by a ¨registered¨ level user will give a JOOMLA error!!! and not a nice ¨not allowed¨ message. Downgrade user acces rights on the category as well if you want to do so.