login failed message - IMAP with TLS

[superfly]

Hi,

I can't login. The message I get is this:
IMAP Error: Authentication for raoul failed (LOGIN): "a001 NO Plaintext authentication disabled."

I'm running dovecot 1.0.beta3 on ubuntu 6.06 server, and I'm using TLS for my authentication. Thunderbird logs in fine, RoundCube doesn't. Where do I specify that RoundCube must use TLS?

Any other ideas?

Thanks!