suddenly we found a problem.
When we go to the admin interface, the libraries appear without owner (only "--" is shown). If changing the owner, after reloading the page the owner disappears again.
In the user browser interface, if we look at the shared libraries (i.e. libraries shared with a group) we get an "Error" message (just that, "Error" in red) instead of the library list.
What could be wrong?
Thanks for your help!