Two suggestions, just shots in the dark:
- I couldn't add a source while using Chrome 117. So it might help to get Chrome on your NAS up to a more recent version. But I have no idea how to do that...
- there's another player in this game, namely the server(s) on the Xfinity side. They may have changed something which made my add-source suddenly work. Try adding the source every day or so. Even if you get errors, the other side is probably getting errors also in their logs. That may motivate them to change something.
Good luck!