i'm still having the problem after that procedure
OS: Win7 Home Premium (German)
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 )
the names of the folders contain spaces, umlauts and 
after creating some folders with these chars in another account and not running into this problem i assume it has nothing to do with the folder's names, either but with the actual amount of folders or messages.
is there a way to mass-rename zimbra folders?
is there a limit of max. folders or messages in either ZD or IMAP?
the webinterface works well but i remember running into similarly problems migrating the users local folders from thunderbird into his zimbra account over IMAP.