Check your .cfg file where you have the fields defined.
Make sure they are numbered consecutively.
Also check to be sure you have the
$auth_user_field = 1;
set correctly for your user id field.
It's hard to tell what could be happening without seeing your .cfg file. Please make a text copy of that file and provide the url where it is located.
Unoffical DBMan FAQ
http://webmagic.hypermart.net/dbman/