Dovecot 2.3.10-2.3.11.3 pop3/imap login problem after upgrade

[Michał Smyczyński]

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