After restarting gnome-shell, when my screen is locked I can no longer enter my password. When I type a character into the password field the character is erased immediately and the message "Authentication Error" is visible all the time (even before starting to type anything).
This is Ubuntu 18.04, Gnome shell 3.28
This turns out to be dependent on how gnome-shell is restarted. I used to do this by typing gnome-shell --replace in a gnome terminal. It turns out that commands run from gnome-terminal are not considered part of the session, but of the gnome-terminal service and this causes the problem. See loginctl user-status for how your processes are grouped into sessions and services, or systemd-cgls for al processes in the system.
To avoid this, restart gnome-shell using the shortcut Alt+F2 r (Alt-F2 to open the run dialog, type r as command, enter), or run gnome-shell --replace from a terminal that does not run in its own service, such as xterm.
Edit: apparently gnome-shell --replace won't work anymore on Ubuntu 18.10+. See here