Some of the modifications made by our modding community are not updated to the latest version of our game. On the latest version of our game, we fixed a bug that caused some of the Steam Issued Activation Keys not working properly. If you are trying to play a modification that is not up to date or requires the player to install an older version of the game, the new keys are not going to work. It is up to the mod team to update the modification.
If you are trying to play our game with a modification, please make sure that the modification is updated to its latest version. Some modifications require additional programs. These programs (or modification itself) can sometimes block the activation process.
Unfortunately, we can't provide any support for issues that occur with modifications for our games. Your best chance of resolving an issue with a mod would be to contact the mod developer directly, or by creating a post explaining the issue on our forums in the relevant section.
We are sorry that we can't be of more help to you with this issue and we hope that you find a way to fix the problem.
We hope this information helps!
If you have any other questions, please let us know - we'll do our best to assist you.