I found a way to get the reports to run! I, too, "upgraded" to 2021 from 2018 Pro and found I couldn't get my memorized reports to run when choosing them from the Reports menu. However...I just now got them to run by going to the Memorized Report List (accessed via Reports/Memorized Reports menu) and double-clicking on a report from there. 

They still don't run from the Reports menu, but at least I don't have recreate them from scratch. I just run from the Memorized Report List, change something minor so that it asks me if I want to save and then I can save it into another group folder. Yea!