I'm also having this same issue.
I have three portrait monitors with res 3240x1920 and when I start the game I get "Failed to create D3D device: unable to find requested mode (800x600@0)"
I've edited GlobalSettings.cfg but still have the problem. I don't see offline\player1.cxl in my %AppData%\Local\Focus Home Interactive\Cities XL 2012\live\ directory I only have playerprofile and shadercache directories. Maybe that doesn't matter since the OP said it didn't help.
So what to try next?