[horde] Problem with umlauts in signature

[Jens Grüntjes]

Hello list,

after upgrading from Horde 4.x to Horde 5.0.0 and Imp 6.0.0 the  
signature of a colleague of mine looks strange concerning german  
umlauts. The signature should be

Ebert + Grüntjes GbR
Michael Ebert, Jens Grüntjes

but in a mail from my colleague this appears in my Inbox (as well as  
in her sent-mail folder) as

Ebert + Grüntjes GbR
Michael Ebert, Jens Grüntjes

It looks like the ü is wrongly encoded (like a UTF-8 character shown  
in latin1). In the Preferences the ü is shown correctly. My colleague  
uses Windows 7 and tried to change the signature in Firefox 16 and IE  
9 but no change. I uses almost the same signature but mine is shown  
correctly as well in Inbox as in sent-mail. I use Linux and Firefox 16.

Can somebody help me?
-- 
Viele Grüße
Jens