Antoine this is probably not the right place to ask a question about a specific game, but I'll try to help you out as much as I can.
The only Solus-related thing that might be causing this is Linux Steam Integration. Open that app up and try toggling the "Use native runtime" option.
Other than that, this is probably a question you should ask in channels more closely related to your game. I'm sure Crusader Kings has a discord or a subreddit where you could get some help?