Dovecot stops accepting connections after Debian upgrade

This had me puzzled for hours....

After an upgrade my Dovecot / Exim4 / Squirrelmail config stopped accepting IMAP connections.

Squirrelmail started giving the following error:

Error connecting to IMAP server: localhost.
111 : Connection refused

After much detecting I discovered that during the upgrade, Dovecot appears to have interpreted the wildcard port directive [::] in /etc/dovecot.conf as strictly IPV6 rather than both IPV6 and IP4. Changing it to a * solved the problem.