As of yesterday 4/23/20 The Mezzmo App on our Roku Ultra has also stopped working. It does work on the older model Roku in the bedroom, but NOT on the Roku Ultra.
I did deleted the cache on the Roku (and restarted) = No effect - No Fix
Then I deleted and reinstalled = No effect - No Fix
Then I reinstalled the Beta version = No effect - No Fix

While the Roku Media app does work on some video - poorly on some videos and not at all on others (I am sure it is an encoding issue).
Hopefully you can get the Roku Mezzmo app updated & working, it really does work So Well! Thanks!