Like Dimitry, I have experienced the same (or a similar) bug on a fresh 20.04 install. I'm attaching a picture of it. Both "Authenticate" and "Cancel" buttons are clickable ("Authenticate" only once) but don't make the window vanish. I confirm that killing the gnome-shell caused the window to disappear.
** Attachment added: "image of the bug" https://bugs.launchpad.net/ubuntu/+source/gnome-shell/+bug/1824874/+attachment/5364530/+files/bug.png -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1824874 Title: undismissable, unclickable authentication dialog left on screen (top- left corner) after policykit authentication [pushModal: invocation of begin_modal failed] To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/gnome-shell/+bug/1824874/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs