Hello,
I have an hMailServer installation that serves emails. I am unable to establish encrypted connections to this server with roundcube. It gives an unknown error if I attempt to use an encrypted connection. The server side is not my suspect because I can establish secure connections using other software such as outlook.
The certificates are proper Lets Encrypt certs and are not self-signed.
Have a look at this https://github.com/roundcube/roundcubemail/wiki/FAQ#problems-connecting-imapsmtp-server-via-ssltls