[horde] Help setting up passwd authentication

[dave--uk]

Hi.  I'm trying to set up Horde on a Centos-4 box using passwd
authentication.  If I set Horde up to use a fixed username and
password it works.  When I change it to use passwd authentication &
login (using the same credentials) it fails: I just get represented
with the login screen, with no error message displayed.  The log file
contains:

Dec 18 11:20:33 HORDE [error] [horde] FAILED LOGIN for testuser
[xxx.xxx.xxx.xxx] to Horde [on line 116 of
"/usr/share/horde/login.php"]

so obviously something isn't right.  I've tried it using the
/etc/passwd file (which is obviously correct) and I know the
credentials are right, but I still get the same error.  I've also
created a separate passwd file but that didn't help.

I've searched the the docs and FAQ but can't see where to go with it.
PLEASE could someone give me a pointer as to where/what the problem
(and solution) might be?

TIA

Dave