[Dovecot] SSL / TLS Problem

Hi,

since I upgraded to the new Thunderbird version 3.1 I can't establish a TLS/SSL connection anymore. But before the update Thunerbird was able to establish an encrypted session ...

Maillog shows me the following now:

server dovecot: imap-login: Disconnected (no auth attempts): rip=84.157.133.248, lip=192.168.1.100, TLS handshaking: SSL_accept() failed: error:14094418:SSL routines:SSL3_READ_BYTES:tlsv1 alert unknown ca

Why is the CA unknowen - all of a sudden - ?!

Thanks