I'm not sure if this is the same issue directly. I've gotten XIV working via Proton, but I had to follow well-known wine answers to get it playable. Namely, I have to edit two files in the steamapps/compatdata/39210/pfx/drive_c/users/steamuser/My Documents/My Games/FINAL FANTASY XIV - A Realm Reborn/ directory.

1. Ff14 Boot Error 30605 504
2. Ffxiv Boot Up Error

Feb 15, 2014 - No matter what I do I can't boot up FFXIV and that error pops up every time. I can't even get to the launcher for crying out loud. Fix: FFXIV ‘unable to download patch files’ If the issue is with your Computer or a Laptop you should try using Reimage Plus which can scan the repositories and replace corrupt and missing files. This works in most cases, where the issue is originated due to a system corruption.

In FFXIV_BOOT.cfg, I had to edit BrowserType to 2, and in FFXIV.cfg I have to edit CutsceneMovieOpening to 1.

Ff14 Boot Error 30605 504

The first change allows me to get to the launcher at this point -- if it's left to its default value I get 'A system error has occured: 404. HTTPS System Error'. Afraid I did the install mid-week, so I'm not sure if this is how I got past nstgc's issue during installation.
Obviously the latter change means I don't get to see the opening cutscene the first time I play the game, but if I leave it at default value, the game launches but hangs up after selecting a Data Center.

Ffxiv Boot Up Error

As these edit game configuration files I'm not sure if this is something Valve wants to consider for Proton, but at the least it's information.