Hi,
I had the same problem, so I changed in the .cfg file the number of the field that refers to the userid in the name of the field:

change
$auth_user_field = 18;
in
$auth_user_field = Userid;

I don't know why, but that fix the problem.
BTW, I've noticed in my MySQL database, that in some routines, the database start counting by field 1,
instead of the flat dbman, whitch started with field 0

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Mart.