It did not help, even when changing permissions in both ".config/Unity3d/sons of welder/Magnus Imago" and also ".config/itch/apps/mangus-imago". I also have Mangus Failure, and it seems to work fine. I notice the the two games store corresponding files differently (in different sub-folders).
Fortunately, it is a relatively short game. I was able to play the game all the way through. Despite not being able to save, I enjoyed it! I would definitely buy it again.