This is really weird. I started seeing the same thing a couple of days ago, and switching Proton versions (latest/experimental, or even to wine) didn’t help. I also bit the bullet of completely reinstalling the game, without success.
The common “fix” for this posted online seems to be to change your DNS server or use a VPN. I’ve looked at my DNS traffic, it’s fine. Haven’t gotten around to trying a VPN yet. (the hypothesis being that rockstar IP-banned you for some reason. doesn’t make sense either, because I’m able to log in to the launcher initially)