I managed to open the "Edit Motion Layer" window once, but now it will no longer open. This seems similar to the problem I have with the "Timeline" window: You can select it from the menu but the window won't open. This is with iClone 8.2.1421.1 on Windows 10. 
Happy to upgrade to Windows 11 if it's known to help, but I don't want to upgrade just to see if (hope that) it fixes the problem.
Extremely frustrating not to be able to get such basic things to work.

Note: I have a 3-monitor system. These problems go away if I unplug the 3rd monitor.