Lda fatal: setgid (102 from userdb lookup) failed

Sorry if this is a nubbie question, but I’m getting:

lda(jlbrown@bordo.com.au)<4444>: Fatal: setgid(102 from userdb lookup) failed with euid=501(jlbrown), gid=20(staff), egid=20(staff): Operation not permitted (This binary should probably be called with process group set to 102 instead of 20(staff))

How can I fix this?

(macOS Mojave, Dovecot 2.3.4)

Thanks,

James.