[imp] expired session when opening imp

[Jack the Fish]

Hi,

I have successfully installed Horde 4.0.13 with imp 5.0.17. The 
authentication is configured to let imp handle the login which works 
fine. The mail server is a remote imap server which is hosted by 
domainfactory (http://www.df.eu).

When I log in to Horde I see the dashboard and I see on the left menu 
the imp mail entry. The imap connections seems fine to me because unread 
messages are shown in the left menu (written in bold 'Mail (2)').

If I click on the mail menu entry, the imp mail screen loads, and a 
warning message appears 'The server was unable to generate the message 
list.'. After I click on another link, I return to the login screen 
('session expired').

I have looked into the debug log (with 'DEBUG' priority) and the imap 
backend log but I don't have any clue why the logout happens. The log 
content shows no error. It seems that after reading the messages from 
the current folder a logout is initiated. But I couldn't find any trace 
who is responsible for this and why...

Does anybody have a idea what is happening?

Sascha