GMzH: My Ubuntu is 20.04.
Hey,
we had another user who reported the game not starting up at all. A different behaviour but he was using the same Ubuntu version via GOG so maybe his solution can help?
He uninstalled the game, then removed the /home/{username}/GOG Games/Vagrus The Riven Realms folder completely. After a new install, the game started for him.
If that does not help, maybe try to open the log file located in ~/.config/unity3d/Lost Pilgrims Studio/Vagrus/ to see if it shows any errors or such. If you do not see any, could you please share that with us?
Thanks!