IPv6 auth failed

[Lefteris Tsintjelis]

I am using latest Windows Thunderbird 68.11.0 and dovecot 2.3.10.1_2 
with FreeBSD 12.1-RELEASE-p8. Whenever I enable dovecot IPv6 and I try 
to login I get auth failed.

Aug 09 08:36:11 imap-login: Info: Disconnected: Inactivity (auth failed, 
2 attempts in 180 secs): user=<user at domain.com>, method=CRAM-MD5, 
rip=1:2:3:4::5, lip=1:2:3:6::7, TLS, 
session=<HZeFLGusb8IgAQZ8LNQAEgAAAAAAAAAC>

Switching dovecot back to IPv4 only (listen = *) starts to work as 
expected. Roundcube seems to also have the same problem whenever dovecot 
IPv6 is enabled.