i'm still having the problem after that procedure
i don't even think it has to do anything with our signature 'cause 3 users here are using a long signature with several german umlauts (ä, ü, ß...) and that works fine.
the account where the problem appears has 100s of folders and subfolders.
(yes, the user wants to keep his folders )
OS: Win7 Home Premium (German)
the names of the folders contain spaces, umlauts and 
i think this could be the problem.
is there a way to mass-rename zimbra folders?
is this worth a new thread?