Fatal: setgroups(6) failed: Too many extra groups when authenticating using mysql - macOS 10.13.6

Using mysql for authentication.

Successful username & password gives: Aug 27 15:10:03 master: Info: Dovecot v2.3.2.1 (0719df592) starting up for imap, pop3 (core dumps disabled) Aug 27 15:10:50 imap-login: Info: Login: user=<jlbrown@bordo.com.au>, method=PLAIN, rip=::1, lip=::1, mpid=24924, secured, session=<E+DEvWN0xcoAAAAAAAAAAAAAAAAAAAAB> Aug 27 15:10:50 imap(jlbrown)<24924><E+DEvWN0xcoAAAAAAAAAAAAAAAAAAAAB>: Fatal: setgroups(6) failed: Too many extra groups

Installed using Homebrew on a Mac running High Sierra.

Any idea how to fix this?

Thanks,

James.