English: this is the general forum - not the Geman sub-forum… that's why.
In the meanwhile, I think, I could solve the issue.
First of all thanks for the hint about the advanced settings.
This was something, I had to find out first.
Yet, the main problem - I believe - was, that any new page which is being inserted by a user, has to be checked for being editable by this very group.
Apparently, the user did not check it, hence disabling her/his editing capabilities and only leaving the admin (me) as an editor.
This brings up another option, which, I believe, could be useful for WB in the admin rights for groups and/or pages:
When setting up a new group, there should be a check box which or all pages can be edited by this very group.
OR: under "pages", the groups which shall be able to edit certain pages should be chosen in general.
I had to go to EVERY single page now and check the very group which shall be enabled to edit.
WB seems to assume, that teh admin knows IN ADVANCE which groups to install etc.
The current system is not user friendly, when groups are set up much later after the site is already up and running and already has a complex structure.