dovecot - Oct 2020 - Dovecot 2.3.10-2.3.11.3 pop3/imap login problem after upgrade

Hi, I came across a following issue. After upgrading dovecot from 2.3.10 to
2.3.11.3 pop/imap logins fail.  Auth method is plain/login ,with ssl required
and shadow passdb driver, runnning on Slackware Linux 14.1.  The configuration
in both cases is the same, and in the past, after each uprgade i had no
problems. I did not come across also of any information if upgrading from 2.3.10
to 2.3.11.3 require any significant change to auth configuration.

Dovecot v2.3.10 (0da0eff44) starting up for pop3, imap (core dumps disabled)
pop3-login: Login: user=<test>, method=PLAIN, rip=89.78.12.2,
lip=192.168.100.30, mpid=8109, TLS, session=<7aCIhC6xzehZTgwC>

Dovecot v2.3.11.3 (502c39af9) starting up for pop3, imap (core dumps disabled)
pop3-login: Disconnected (tried to use unsupported auth mechanism):
user=<test>, method=USER, rip=89.78.12.2, lip=192.168.100.30, TLS,
session=<CYwFfS6xouhZTgwC>

I appreciate help if someone knows the cause and solution. 

Regards