sometimes no shared cipher after upgrade from 2.2 to 2.3

We recently upgraded from dovecot 2.2 to 2.3.7.1-1

Not many, but some users are experiencing difficulties. The dovecot directors log:

Aug 21 14:28:49 director01 dovecot: pop3-login: Disconnected (no auth attempts in 0 secs): user=<>, rip=redacted, lip=10.0.0.120, TLS handshaking: SSL_accept() failed: error:1408A0C1:SSL routines:ssl3_get_client_hello:no shared cipher, session=

Any ide what could be causing it?

Thanks, Kristijan