sometimes no shared cipher after upgrade from 2.2 to 2.3

[Kristijan Savic - ratiokontakt GmbH]

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=<EeoTt5+QXwVUl87W>

Any ide what could be causing it?

Thanks,
Kristijan
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