Reproduced it with this hardware / software combo:

Kernel: 6.19.10-1-cachyos-bore
CPU: AMD Ryzen 7 9800X3D @ 5.4 GHz
GPU: GeForce RTX 5090 - nvidia (open source) 595.58.03
Proton: proton-cachyos-10.0-20260321

Steam launch options:
=> LD_PRELOAD="" PROTON_VKD3D_HEAP=1 NVPRESENT_ENABLE_SMOOTH_MOTION=1 MANGOHUD=1 game-performance %command% => GAME LAUNCHES,
=> LD_PRELOAD="" PROTON_VKD3D_HEAP=1 MANGOHUD=1 game-performance %command% => GAME LAUNCHES,
=> LD_PRELOAD="" MANGOHUD=1 game-performance %command% => GAME DOES NOT LAUNCHES

I can provide logs if required.

Thanks, it resolved my Xid 109 CTX SWITCH TIMEOUT issue on The Last of Us part II as well

Operating System: Bazzite 43
KDE Plasma Version: 6.6.3
KDE Frameworks Version: 6.24.0
Qt Version: 6.10.2
Kernel Version: 6.17.7-ba29.fc43.x86_64 (64-bit)
Graphics Platform: Wayland
Processors: 12 × AMD Ryzen 5 3600 6-Core Processor
Memory: 32 Gio of RAM (31.3 Gio usable)
Graphics Processor: NVIDIA GeForce RTX 5060 Ti (nvidia 595.58.03)
Manufacturer: Micro-Star International Co., Ltd.
Product Name: MS-7B89
System Version: 1.0

Anecdotally, the descriptor-heap-test branch actually fixes many more games on the 595 drivers on Blackwell that hit the Xid 109 hard lockup: Ghost of Tsushima, AC Shadows, Elden Ring to name a few.

I've updated the OP with relevant updates:

EDIT: There are many reports below + on Reddit and on Discord that using the VK_EXT_descriptor_heap branch also fixes Xid 109 Hard Crashes. Since it has been massively reported to me by many people, I decided to incorporate this into this issue.

EDIT 2: Conversely, there is a minority of people that this workaround didn't work for them in Crimson Desert right away. I suspect that their game may be left in bad state after trying to run the game without this workaround first. All that I can suggest to people who have been struggling is to clean their steam cache and verify their game files. This shader cleanup has been reported to solve these issues with the workaround most of the time. However, If that still doesn't work, all I can think of is to backup their save and reinstall the game.

I deleted my Steam cache and the PROTON_VKD3D_HEAP=1 launch option still wasn't working, but when I uninstalled the game and reinstalled it, it finally worked!

Operating System: CachyOS
Kernel Version: 6.19.10-1
Graphics Platform: Wayland
Processors: Intel i9 14900KS
Memory: 64GB DDR5 Corsair Dominator Platinum
Graphics Processor: NVIDIA GeForce RTX 5090 -595.58.03 driver
Proton Version: Proton-CachyOS Latest (10.0-20260324 x86_64_v3)

Same Problem with Space Marine 2 but unfortunately, clearing the cache, reinstalling the game, and setting PROTON_VKD3D_HEAP=1 didn't help.
I was able to access the settings once after reinstalling the game, but when I tried to save, the game crashed and wouldn't start again.

Operating System: Arch Linux
Kernel Version: 6.19.10-zen1-1-zen
Graphics Platform: Wayland
Processors: AMD Ryzen 7 9800X3D
Memory: 32GB DDR5
Graphics Processor: NVIDIA GeForce RTX 5080 - 595.58.03 driver
Proton Version: GE-Proton10-34

@Helmchen-Lord

Operating System: Arch Linux Kernel Version: 6.19.10-zen1-1-zen Graphics Platform: Wayland Processors: AMD Ryzen 7 9800X3D Memory: 32GB DDR5 Graphics Processor: NVIDIA GeForce RTX 5080 - 595.58.03 driver Proton Version: GE-Proton10-34

I guess you are using the wrong proton version. As far as I tested the above solution only works with proton-cachyos versions: 10.0-20260321 x86_64 and 10.0-20260324 x86_64.

CachyOs devs baked the new heap testing branch into their proton.
Please correct me if iam wrong.

@xcsvxangelx You are correct. @Helmchen-Lord It requires Proton-CachyOS 10.0-20260321 or newer.

OS + DE: CachyOS, KDE Plasma v6.6.3, Frameworks v6.24.0, Qt v6.11.0
Kernel v6.19.10-1-cachyos (64-bit); Wayland
AMD Ryzen 7 5700X 8-Core Processor
Graphics Processor: NVIDIA GeForce RTX 5060 (nvidia 595.58.03)
RAM: 32Gib

I'm probably among the minority of the minority where almost nothing worked, but after days of tinkering I found a solution for the RTX 5060 (at least it works for me, hopefully it works for any other who's running into similar issues).

The main issue: when upgrading to nvidia driver 595.58.03, Crimson Desert would open normally but would freeze while on the loading screen even though the music kept playing.

Although I am unsure if this only happened to me or if other people are having this issue, after days of tinkering and going back and forth with the drivers, I finally found a solution, at least for my RTX 5060

Steps to fix:

1. Uninstalling the game and reinstalling (without opening)
2. Deleting all game and nvidia cache:

rm -rf ~/.cache/nvidia/GLCache/*
rm -rf ~/.steam/steam/steamapps/shadercache/3321460/
rm -rf ~/.local/share/Steam/steamapps/common/Crimson\ Desert/bin64/vkd3d-proton.cache
rm -rf ~/.local/share/Steam/steamapps/common/Crimson\ Desert/bin64/vkd3d-proton.cache.write

(make sure you know what your steam path is)
3. Upgrading system and drivers, then rebooting
4. Click on Crimson Desert > Properties > Compatibility > Choose either proton-cachyos-10.0-2026-0324-slr-x86_64 or the 0321 version
5. In launch options: PROTON_VKD3D_HEAP=1 VKD3D_CONFIG=no_upload_hvv DXVK_ASYNC=1 %command%

That one command worked for me the first time I opened the game, it had a tiny freeze at the end of the loading screen but it went through and now the game opens and runs normally.