The old app id (480) has been replaced with a new one that can save your online progress. USE THE NEW STEAM APPID (41010).
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How to change steam appid

1. Navigate to your Sniper Elite v2 installation folder
2. click on the "bin" folder
3. Open the "steam_appid.txt" file
4. Replace 480 with 41010
5. Save the file
6. launch Sniper elite v2
7. You should have now tricked steam thinking you are playing Serious Sam the second encounter demo
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NOTE: You can only play with people with the SAME app id as you.