This should not happen under normal circumstances, as per GNOME's security policy, whenever you enter the lock screen all the extensions get disabled automatically until you get back in, and then the extensions get re-enabled. Somehow it's failing here for the Ubuntu Dock extension (gnome-shell-extension-ubuntu-dock) and so it's a bug.
Note: Bug reports are off-topic here.
As a temporary workaround either restart (or relogin to) your system or restart GNOME shell by pressing Alt+f2, then typing r and pressing Enter.
This bug was reported as being fixed in the package gnome-shell-extension-ubuntu-dock, 0.9.1ubuntu18.04.3. Perhaps the bug will go away if you update to that package.
There is a solution, I had the same problem in ubuntu 19.10 (sic!).
And this command fixed it:
gsettings set org.gnome.shell.extensions.dash-to-dock transparency-mode FIXED
This should not happen under normal circumstances, as per GNOME's security policy, whenever you enter the lock screen all the extensions get disabled automatically until you get back in, and then the extensions get re-enabled. Somehow it's failing here for the Ubuntu Dock extension (
gnome-shell-extension-ubuntu-dock) and so it's a bug.Note: Bug reports are off-topic here.
As a temporary workaround either restart (or relogin to) your system or restart GNOME shell by pressing Alt+f2, then typing
rand pressing Enter.This issue is resolved with the following command, found it on this github issue: https://github.com/micheleg/dash-to-dock/issues/649#issuecomment-348433909
NOTE the comments below about how this is hacky and a very undesirable method to solve the issue.
This bug was reported as being fixed in the package gnome-shell-extension-ubuntu-dock, 0.9.1ubuntu18.04.3. Perhaps the bug will go away if you update to that package.
There is a solution, I had the same problem in ubuntu 19.10 (sic!). And this command fixed it: gsettings set org.gnome.shell.extensions.dash-to-dock transparency-mode FIXED