I've just upgraded from 20.04 using UI (not Terminal). Everything was finished without any errors. The OS was rebooted OK. I see the only but severe issue - mouse movement freezes everything. For example, the video is frozen in VLC, while I move mouse across the screen. Windows dragging is extremely laggy. I tried to look at System Monitor, but it doesn't show any CPU usage peaks while I move the mouse.
So, what could be the reason?
I have wireless Logitech mouse, attached to USB2 port of the Pi. I have Pi 4 with 8GB RAM. 20.04 worked without any problems.