Per the Developer, trading .bak files isn't prohibited. But, it's odd. Keep in mind the .bak files are all-or-nothing. So when you get someone's .bak file, you get all their characters, all their equipment and all their quest choices. Since endgame gear can only be obtained in the end-game, that means there's not much left to play.
There's also malware and other bad actor concerns with swapping game files from unknown persons.
I -thought- just grinding in the early game to create a quest-less .back file of medium level to get new players a boost in the early game, but opted against it for the above reasons.
Better advice, is if your stuck in the game, ask for help how to adjust your character or your playing style to progress better.
Best of luck.