Completely disabling gnome extensions stopped the problem for me (not just disabling every extension; the extensions feature itself had to be switched off).
$ gnome-tweaks
Click "Extensions" in left sidebar
Turn the switch at the top right OFF
Log out & log in again (or reboot)
BTW, Alt-F2, restart can get out of the freeze once or twice.
Update 3/30/2020: There is something wrong with the remnants of old extensions, even if all extensions are disabled! Probably some gnome-shell bug or version-incompatibility corrupted things (just a guess). Anyway, I completely fixed the problem by doing
rm -rf $HOME/.local/share/gnome-shell
while logged non-graphically on a vt (log out and wait for the login screen, then Ctl-Alt-F5 for a vt; Ctl-Alt-F1 to return to graphical login)
(I also removed .local/share/gnome-settings-daemon/ and session_migration-ubuntu but don't really know if that was necessary -- it's probably best to just remove .local/share/gnome-shell at first.)
Then log in graphically, re-enable gnome extensions using gnome-tweaks, and check if the problem is gone. If so, re-install any non-stock extensions from https://extensions.gnome.org/
P.S. FWIW, the "Dash to Dock" extension seemed to provoke problems, so I did not want to re-install it;, I tried "Workspaces thumbnails" instead and am happy with it.