When running the ThinLinc client in fullscreen on macOS you shouldn't unplug a monitor apparently. Doing so makes both screens go completely black. Plugging the screen back in again does not help. Pressing F8 does not change anything.
The only way I found to get out of this was to disconnect the ThinLinc client, either by replacing the connection from a different client machine, or by blindly pressing F8, then arrow down -> ENTER to select "disconnect" in the F8-menu.
*** This bug has been marked as a duplicate of bug 7160 ***