tlsv1 alert unknown ca: SSL alert number 48

Hi,

I have set up a mail server with postfix+dovecot 2.2.13 on my raspberry pi running Raspbian Jassie OS.

Now I would like to add an on-line e-mail client like Squirrelmail or Roundcube. I was able to start up these two clients but when I try to login I get this error message in the dovecot log:

tlsv1 alert unknown ca: SSL alert number 48

But I have inserted the self-signed certificate and key in /etc/dovecot/conf.d/10-master.conf

Moreover, I can send and receive e-mails from/to my server, and I can login successfully to dovecot IMAP with Thunderbird.

Can somebody give me a clue on how to solve this problem? Any help would me much appreciated.

Regards, Maurizio