passdb doesn't support credential lookups

I'm having trouble authenticating certain users. I see this in the logs:

Nov 19 16:26:40 auth: Debug: pam(jane,192.168.2.83,<...>): Performing passdb lookup Nov 19 16:26:40 auth: Debug: pam(jane,192.168.2.83,<...>): passdb doesn't support credential lookups Nov 19 16:26:40 auth: Debug: pam(jane,192.168.2.83,<...>): Finished passdb lookup

at the command line, if I run this:

% doveadm auth login jane Password: passdb: jane auth succeeded

Oddly, I have a handfull of users that are failing and others that are not and I don't see the difference.