[imp] MIME parts are garbled

[Martin]

I have a site up running horde 3.0.6, imp 4.0.4, and some other modules.

I have just upgraded to horde 3.1 and imp 4.1. After that everything still 
works well, only mails are not correctly displayed anymore: all MIME parts 
are garbled.

A mail with just one text/plain part
    Body shows: "Return-Path: Received: from [..]"
    It contains the whole header again, but nothing else.

A mail containing some plain text and an attachment
    Parts:      1  unnamed [CERT";i:1/mixed]  	0 K
    Body shows: "There are no parts that can be displayed inline."
    In folder few the attachment flag is not set.

Do you have an idea where to start looking for an error in my configs?
Did I miss anything while upgrading? The old site is still up and running 
fine.

Thanks,
Martin