DS 1.5 Created Folder for Bookmarks are invisible

I have a customer DS Installation - BOE 4.1.6 DS  1.5 - where the customer is unable to use bookmarkfolder within the CMC.

They get created but stay invisible. When you try to create Folder "test" again, it tells you that this Folder already exists. But the Folder itself stays invisible - also in the move to menue for the Bookmarks no Folders are visible!

Any ideas?

Wobi