I just registered in the forum to post about this issue that happened to me twice. It happened the same way, after an update, upon reboot I end up with a black screen with a functional mouse after login. I still can spawn windows and open the launch dialog with ALT+SPACE but no desktop background, menu or icons whatsoever.
The first time I couldn’t “fix” it despite searching for solutions and had to do a fresh install.
This time happened again but this time I tried something different. I had two monitors attached, one through HDMI and the other through VGA, both on board graphics (this is a Ryzen desktop), so I unplugged the VGA monitor and rebooted. After reboot the desktop came back again and after pluging it back and fiddling with the settings I’m now back to a working environment as before the update. Kernel 5.9
I hope this helps any poor soul that suffers from this bug.