Yesterday I took the time to at last play a longer session (more on balance another time :D). After chosing to end the game the screen turned black and the mouse pointer stayed "ingame". Pressing the Windows key made the task bar show up, but clicking any other window made the screen turn black again with ingame mouse pointer.
I opened the task manager by keyboard shortcut (system was working) and using the "preivew" function of Windows (hovering over taskbar, then over the window preview) I could see the process "Guild 3" in the list, but stated to be "inactive". After blindly managing to highlight the process and killing it the system went back to normal.

System specs:
Ryzen 3700X
Radeon 5700XT
16GB RAM