When using external authentication I found out that the authentication module did not retrieve the database fields from the server.

I had to change line 110 in auth/db/lib.php from:
$result["$field"] = addslashes(stripslashes($rs->fields[0]));

to:
$result["$field"] = addslashes(stripslashes($rs->fields[$pcfg["field_map_$field"]]));

After that authentication with the external database worked fine.

I also had a similar problem in the database enrolment module (enrol/database/enrol.php) where I also had to change the field access from index to id.

http://moodle.org/mod/forum/discuss.php?d=59540