What does the errorcode 403 mean?
The errorcode 403 means Access Forbidden.
How can I resolve the problem?
Check if you have installed Akeeba Admin Tools, if yes, the possible cause is that the CSRF shield is activated. This needs to be turned off. Further in the htaccess creator, in the security section, the folder /cli must be added to the Allow direct access, including.php files, in these folders. After this RO Payments should work again.