Hello,Update:
The issue was caused by the REDlauncher not running on Steam Deck (unlike Cyberpunk 2077, which still launches with the REDlauncher). The Witcher 3 doesn't — which meant I couldn’t log into my CD PROJEKT RED account and sync my saves. I ended up resolving it by launching The Witcher 3 on PC, where the launcher works fine and cloud save sync happens quickly and smoothly. Please consider bringing the REDlauncher back for The Witcher 3 on Steam Deck — it would really help with cross-progression support.
So, REDlauncher should be actually launched before The Witcher 3 - as it is part of the game build. I could imagine couple of possible causes of not launching it:
1. There are some launch flags/options passed during launching and because of them, REDlauncher is actually skipped.
Please, make sure, that on SteamDeck, in game Preferences -> General, the "Launch Options" field is empty.
2. There is some Game files integrity problem.
Please navigate to Preferences->Installed Files and use "Verify integrity of game files" option.
3. REDlauncher fails to be launched (or crashes).
If such happens, there are two things:
3a) It would be great, if we would get the generated REDlauncher logs from you. We could analyse the issue and maybe fix potential bug.
To get the logs, you need to switch to "Desktop Mode". To do that, you can hold power button (or click "STEAM" button and choose Power). In the menu that appeared, you should choose "Switch to Desktop".
Now, "Desktop mode" allows you to navigate through the entire system. The logs are located here:
/home/deck/.steam/steam/steamapps/compatdata/292030/pfx/drive_c/users/steamuser/AppData/Local/Programs/CD Projekt Red/Redlauncher/logs/*
You could then either use the browser on SteamDeck to send them, or use extra software (like warpinator) or configure scp to get the files from the device to PC
3b) Maybe the REDlauncher files got corrupted. You could try to remove them (after removal, during the next Witcher3 launch, the REDlauncher files should be recreated).
To remove the files, again you need to be in the "Desktop Mode".
You should just remove the whole directory
/home/deck/.steam/steam/steamapps/compatdata/292030/pfx/drive_c/users/steamuser/AppData/Local/Programs/CD Projekt Red/Redlauncher/
After the removal, you should just get back to "Game Mode". It could be done by double-clicking on "Return to Gaming Mode" icon on the desktop on SteamDeck.
Now you can launcher Witcher3 again and observe.