NEW HYPOTHESIS:
I'm seeing failures even when the SiriusXM app should already be loaded. One of the features of the Roku Ultra is lots of memory so you can keep multiple apps loaded at once. However, as renojim pointed out, the Roku box still has to communicate with the SiriusXM server for authentication. It's possible that loading and authentication, or even just authentication, are considered part of the loading process. An inability to authenticate might be reported as a "loading" error, even though the app has already been loaded. It's not ready to run until you have established your right to access the SiriusXM programs, thus it's still "loading".
That will be something else to talk to SiriusXM about next week.
SOLUTION, PROBABLY:
I spent about an hour on the phone today with SiriusXM Tech Support. It was one of the best tech support experiences that I've had. The technician assumed an intelligent customer and did not simply follow a trouble-shooting script, nor did he ask me to repeat steps that had already been attempted. When I described the problem, he already seemed to have a pretty good idea what the solution was.
It was a SiriusXM authentication problem, possibly a conflict between multiple streaming devices. Just logging in each time using the same user ID and password didn't work. What we had to do was remove and then re-install the SiriusXM app, and then re-authenticate via the SiriusXM web site using the authentication code provided when the app re-started. That worked. All of our personal information came back (e.g., favorite channels), and we're now able to move between Roku channels without having to login to SiriusXM each time we return.
I've used the word "possibly" because I can't recall any other streaming use of SiriusXM in the time frame where we experienced the problem. Not saying it didn't happen, but I don't recall it. Regardless of the cause, it was clearly an authentication problem and my hypothesis about hanging while attempting to authenticate use of the SiriusXM app was probably close to the truth. This is also someone's software problem because it should have timed out with a "failure to authenticate" error message. That would have been much more enlightening and would have immediately suggested possible solutions.