If you are not able to login as a client from the admin panel of the WHMCS, this is the solution,
Ensure you are logging into your admin area using the exact URL defined in Setup > General Settings.
If for example your WHMCS System URL is set to http://www.example.com/whmcs/, you would need to login to your admin area at http://www.example.com/whmcs/admin. If you login at http://example.com/whmcs/admin this can cause this problem (note the www. was omitted).
