WHMCS Invalid Csrf Protection Token Error

 You can disable CSRF Checks under Setup > General Settings > Security.

Setup > General Settings > Security tab

Or from phpmyadmin, run this SQL statement:

UPDATE `tblconfiguration` SET `value` = 'on' WEHRE `setting` = 'DisableSessionIPCheck'

Okay, so I managed to fix it by switching to other browser, disabling CSRF Checks and then enabling them again.