This guide is a continuation on Guide: Installing C&C Generals. If you have not followed that guide to install your game, then there is a small chance this trick won't work for you.
Below you will find the YouTube video guide on how to fix the 'serial in use' issue when playing via LAN. This guide works for all versions of Windows 10, Windows 7, Windows Vista, and Windows XP.
If you have any questions, please write them as comments on the YouTube video so I can answer you. Click here to watch the video on YouTube.
It's recommended to change the last two digits of your serial number using the following random generator:
After applying this trick you might have your base blow up after 30 seconds. To test this, start your game, go to Multiplayer -> LAN -> Create Game -> Play Game. Once you have completed building a War Factory / Arms Dealer and your base has not blown up, then you are ready.
If your base does blow up, download DatGen and place it in your installation directory. Now rename generals.dat to generals.dat.bak and run DatGen as administrator. Now start the game and try again. Your problem should now be resolved. If you still have the same issue try changing the serial again.
If you still encounter the bug, try installing the game with a different serial number to see if that fixes it. Then you can always repeat these steps again and it should work.
If you generated a new Generals.dat successfully and still encounter the 'Your serial is in use' when playing online via GameRanger, then just keep re-joining the room. Eventually you will be able to join. If the problem persists try changing the host in GameRanger. This is a connection issue, not a serial issue.