"Bad serial number" SOLVED

This happens because for some ♥♥♥♥♥♥♥♥ reason the game stores the serial number of your game in "ArmA 2" folder when it should have it stored in "coldwarassault" folder instead.

Solution:
1) Press Win + R
2) Type "regedit" and press Ok
2) Go to HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\bohemia interactive studio\ArmA 2
2) Open "KEY" (It should look like "FB B4 EC GH 43...")
3) Take a screenshot of it (because copying won't work for some reason)
4) Open HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\bohemia interactive studio\coldwarassault
5) Press right click -> create Binary Value
6) Name it "KEY" and open it via double click
7) Write the BINARY serial number manually from the screenshot you've made (from step 2) and press OK (It should look the same as in ArmA 2 folder).

Done! Have a great day.