Unable to accept SSL certificate (k9mail)

I am setting up k9mail on a 4.2 device (rooted Sony Xperia Mini Pro with stock ROM and crap removed) and it complains about the IMAP SSL certificate not being valid. This happens with both a self-signed certificate, and also on a different IMAP server with Comodo-signed certificate (which is normally accepted by browsers and other mail clients). > Unrecognized Certificate > > The server presented an invalid SSL certificate. Sometimes, this is > because of server misconfiguration. Sometimes it is because someone is > trying to atta you or your mail server. If you you're not sure what to > do, click Reject and contact the folks who manage your mail server. > > (java.security.cert.CertPathValidatorException: Trust anchor for > certification path not found.) Certificate chain: > > ... Clicking "Accept Key", however, make the same dialogue come up again and again in a loop. What is the problem? Can it be that k9mail is unable to add the key to the cert store? How to debug this?