I have the same problem: "cannot create save data" when I try to save a campaign. I am not using Bitdefender but Microsoft Security. I do not think it is the antivirus software as I tried turning off the anti-virus but still cannot save a campaign. Tried reinstalling and also making sure all the relevant folders have write access.
The game does automaticaly create a saved game but if I try to load it, the game crashes. Other steam games work fine.
Any suggestions would be appreciated.