Last night I tried to run the Mezzmo app on a Ruko 3 (4200X) and it did not work. When running the app it will display the Mezzmo starting icon for about 2 seconds and then drops back to the Roku home screen. This is repeatable. In trying to debug this I have:
1) restarted the Mezzmo server and PC
2) restarted the Roku box
3) removed the Mezzmo app and reinstalled it

Other points are
1) The two LG smart TVs can find and play off the Mezzmo server
2) The Roku TV (Cheap BestBuy TV) can find and play off the Mezzmo server
3) I have been running the Roku 3 for years without any issues

I did look and there was a software update to the Roku 3 box on April 15, 2020. Version 9.3.0 build 4118. So far this update to the Roku 3 looks to be the I only thing I can find that has caused this problem.

Note: I just hooked up an used Roku 3 to the same system with software version 9.2 and that still has no problem running the Mezzmo app.