Re: Dragon Age Origins Ultimate DLCs not working on Xbox Series X

Got it to work!

1. Install only using the second disc containing the DLC, make sure all DLC is installed (check your installation queue)

2. Clear your Xbox 360 storage (Settings > System > Storage > Clear local Xbox 360 storage) [current workaround for backward compatibility issues according to "Known issues on Xbox Series X|S" in the Microsoft forums]

3. insert the second disc and start the game

The second step is what made the difference for me.