dovecot userdb Fatal imap setuid errors

[essandess (Steve Smith)]

I'm following the MacPorts guide to set up dovecot user and password databases on Mojave, ​https://trac.macports.org/wiki/howto/SetupDovecot for a multi-user system. I am able to authenticate successfully, and doveadm user myusername works for all accounts.

However, dovecot's mail-err.log shows setuid issues when multiple users are logged in:

May 21 22:28:46 imap(pid 17441 user myuser): Fatal: setuid(512(myuser) from userdb lookup) failed with euid=501(adminuser): Operation not permitted (This binary should probably be called with process user set to 512(myuser) instead of 501(adminuser))

I do not see the cause pf the issue. I've double checked everything and have not used either myuser or admin user in dovecot's /opt/local/conf.d or indeed anywhere in /opt/local.

My passdb userdb blocks are simple:

passdb pam {
  driver = pam
}

userdb passwd {
  driver = passwd
}