Easier workaround (for me) is to restart the Roku device.
I have found (for me) a much better work-around then reinstalling the channel (app). It is to restart the Roku device and then go to the Great Courses Plus app. The Roku can be reset by going into the Roku Settings, then System, then System Restart. On my Roku Ultra it doesn't take long to restart.
I just recently subscribed to Great Courses Plus, and still see this problem, as of Feb 2021. So of course I should have the latest version of the Great Courses Plus channel (app). I have seen this on multiple Roku devices (Ultras). I had previously used Great Course (NOT Plus) which is a different Roku channel (app), and I don't recall having this problem.
To add a little detail, here is what I experience. The Great Courses Plus channel (app) will come up, and I get the spinning icon. Sometimes, I can access the "category" menu and find the course I want to view, and it will work. But other menus, such as the "watch list" menu, just hang (spinning icon). I have also seen cases where, after a few minutes, things start to work.
I have reported this to Roku, and mentioned that I cannot recommend the Great Courses Plus service for people using Roku.