Assassin’s Creed 2 uses an old Ubisoft DRM system requiring an online check-in. Even for offline play. If Ubisoft’s legacy servers lag or your connection hiccups, you get error 21—blocked from your own save file.
: In late April 2010, the scene group Skidrow released a crack that bypassed the DRM check entirely without needing a server emulator. This allowed for a "single file" replacement (typically an .exe or .dll ) that made the game fully playable offline. The Official Resolution assassins creed 2 offline server 21 single file cracked