The API is still in beta so there may be a few little problems here and there. The XRDS is only for auto-discovery and shouldn’t affect whether or not you are able to add the application.
Perhaps you could try to add the application again and then go to the logs to see if there is anything in there that might give an idea what is wrong.
I just added new application without any problems.
Could you go to Advanced->Report Fault page, click on “Switch to Test Mode”, recreate your issue and tell or even better show us what happens?
After you are done with it, remember to switch it back to production mode.