Urmas,
Looking at your image for Error Code - 501 (Not Implemented), I understand that when a certain feature has not been implemented within the CA Clarity PPM, the Error Code - 501 is what gets displayed. In our case, it is related to the XOG Client feature that has not been implemented and in some other cases, it may be related to that particular object or feature, which has not been implemented within the CA Clarity PPM.
On the contrary, Error Code - 503 (Service Unavailable) could be related to a particular CA Clarity service or feature that has been made unavailable. It may be present within the database, but, it may have been disabled by the system administrator. So, there could be a possibility that as Georgy highlighted earlier about a way to enable the XOG Client on the CA Clarity PPM v13, but, it may be possible that after enabling this feature on v13, it may have been disabled by the system administrator. Hence, the Error Code - 503.
Suhail.