Unable to save in current version of game. Completely. Even when game says, that its saving - it doesnt (if you even going back to main menu - thats it, your progress is lost forever).

Thank you for letting me know about this. We are currently working with GOG to get this figured out.

If it helps, console log (in wine) says:

PLAYER_INFO::save_file() open operation failed.

(Then again, it's not an awful lot of info)