Ext Auth does what it should but eJabberd says 'noexists,passwd'

I have been running ejabberd on a Linux machine happily for some time now but recently moved to a FreeBSD box. I cannot seem to get ejabberd up and running with external auth. The auth script runs and has the exact same output as on the Linux machine - it seems to be working properly. But ejabberd is still complaining with the following cryptic message:

** Reason for termination = 
** {{badmatch,{aborted,{no_exists,passwd}}},
    [{ejabberd_auth_internal,set_password,3},
     {ejabberd_auth_external,check_password_external_cache,3},
     {ejabberd_auth,check_password_loop,2},
     {cyrsasl_plain,mech_step,2},
     {cyrsasl,server_step,2},
     {ejabberd_c2s,wait_for_feature_request,2},
     {p1_fsm,handle_msg,10},
     {proc_lib,init_p_do_apply,3}]}

This, to me, seems to indicate that there's something weird about the passwd file?

ejabberd:*:543:543:ejabberd pseudo user:/var/spool/ejabberd:/bin/sh

That doesn't seem unusual to me.

So what is this error message really saying? Any help would be appreciated.

Linux was running ejabberd 2.1.2 while FreeBSD is from 2.1.5