If your game crashes whenever you try to play Online Mode, block all your friends. You don't have to delete them, just block them and then unblock them when you're done.

If you go to
https://gtm.steamproxy.vip/id/<your username>/friends/all
you can easily block/unblock all your friends.

The game will still crash, but at least for me it usually happened after I had finished the stage, not before.

I tried pretty much every other solution and this was the only one that worked for me.

Thanks to Immortal King for the tip!