Medicallucky

Arma 2 exited with 'Bad serial number given in setup' error after a few seconds after launch. When I opened 'regedit', I saw that field 'Key' in 'HKEY_LOCAL_MACHINESoftwarebohemia interactive studioarma 2' is empty. Any workarounds suggested for windows version of the Steam and Arma2 had no effect in my case. Arma2: Operation Arrowhead works well.
The only way for me to get it works is: install win-version of steam and arma2 in wine and copy the registry entry 'Key' to Proton prefix through Proton regedit. But it resets every time the game updating.

Same here.

The following worked for me:

1. I used the Sethioz Industries ArmA 2 key conversion tool to convert my Product Key into Hex.
2. Below, I assume these environment variables. Reorganise them as you see fit.
3. I edited that value into the Windows Registry using ${WINE} regedit with WINEPREFIX as above. You need to edit HKEY_LOCAL_MACHINESoftwarebohemia interactive studioarma 2, key Key. You should find it empty before you edit.
4. Don't run the game via Steam. When Steam launches the game, it clears the key from the registry for some reason.
5. Run the game using ${WINE} ${SA}/common/Arma 2/arma2.exe, with WINEPREFIX as above.

I also observed that during the installation of the game, the installer displays the Product Key for you to copy and then it opens the Explorer window in the 'My Computer' view. That's strange because I don't know what to do there and I have to close the Explorer window to let the installer continue. I suppose that this is a result of some installer script gone wrong and that instead of prompting you for the Product Key, it opens Explorer instead, which leaves the key empty in the Steam configuration. Steam then saves this empty key into the registry every time you run the game, I guess.