[Dovecot] Problems with Mozilla

[Tres!]

Hello,

I've some problems with Dovecot and Mozilla.
First of all i have to mention, that Dovecot is installed on SuSE Linux 
8.2 and uses up to 20 Win32 Mozilla clients. If you need further 
information, please let me know.

My problem is, that I got one or two times a week a login failure from 
Mozilla, and the clients can't connect anymore. The error message from 
Mozilla after a long time "checking server capabilities" is "server is 
not an imap4 server".
I haven't found something usable in the logs except for a login timeout 
of the clients who want to connect. If i kill the imap and imap-login 
tasks and restart redove, everything works well for some days. Is there 
anyone who knows something about my problem?

regards
Tres!