Update 21:17: Looks like Proton developers have updated the "bleeding-edge" Beta for Proton Experimental, that works around the issue. Note: using it can cause other issues, the Beta doesn't have a lot of testing, you've been warned.
If you go into the Properties on Proton Experimental in your Steam Library, and opt into the "bleeding-edge" Beta, then set your Ubisoft games to use Proton Experimental it will fix it for now until a proper Proton update is out.
Update 22:11: Valve released a Proton Experimental (no Beta needed) fix for this.
Original article below
Third-party launchers on Steam are once again being a massive nuisance. First it was EA breaking everything on Linux and Steam Deck and now it's Ubisoft telling everyone to hold their beer.
Valve fixed the EA App in Proton after a while, and it's likely we're going to need to wait on a fix from Valve again for whatever Ubisoft changed in the latest Ubisoft Connect update.
When you go to launch any game on desktop Linux or Steam Deck that uses Ubisoft Connect, it will do an update and then you'll be greeted with this:
That picture above is from my Fedora Linux desktop trying Ghost Recon Breakpoint today.
After testing The Division 2, Watch Dogs Legion, Assassin's Creed Valhalla and Ghost Recon Breakpoint on my Steam Deck too, they all also gave the exact same problem as you'll see in the screenshot below from my Steam Deck:
So it seems that every title that uses Ubisoft Connect, is now left broken on Linux desktop and Steam Deck. This comes only two weeks after Ubisoft fixed The Division 2 for Steam Deck / Linux. Meanwhile, Breakpoint still needs manual fixes.
Why do developers and publishers keep forcing these absolutely useless third-party launchers on us? Never once have I, or anyone I've spoken to, actually wanted them. They only ever cause problems and solve basically nothing that Steam cannot already do directly.
I've logged an official bug report with Valve and notified their developers about it.