Thanks for your feedback, please download the WarPlan-Hotfix.zip attached and follow the instructions to fix the Invalid serial issue.
There is a set of instructions within the zip for you to follow, please do so
Summarised below too!
Unzip folder to somewhere on your computer (this can be on your desktop for example)
Inside unzipped folder - Select all contents of the folder and copy to the root install folder of WarPlan (where you installed the game to)
It should ask you to replace 4 files, please do so.
Before running the game, make sure to delete the "playerData.dat" file if it is present in your "Documents\My Games\WarPlan" folder.
Thank you
Sam

