Webmaster2020.com

I have the following alert in my logs /va/log messages
from dovecot. The client is also not showing new messages.
Client software is k-9 mail on Android.

Sounds like the client is not accepting the certificate.
did you change the certificate of the server recently ?

I think you need to accept the new certificate on the client software.
In K-9 it is in Settings > accounts > Fetching mail > Incoming Server > Next of Advanced
and accept the new certificate.

Yes, indeed accepting the new certificate solved the problem.
But should K-9 not show a warning when the SSL-certificate changes ?
instead of doing nothing ? It is confusing.