Try going into Steam Library, right click MW3 Multiplayer and go to Properties > 'Local Files' tab > click the button 'Verify Integrity of Game Cache'... it will verify all files and re-download what you are missing... there is a possibility something is corrupt as this sounds like a 'file missing/invalid' type error... this may also be a language support error, which I'll admit I know nothing about if you are using a different game version with Italian language support (or Russian, Chinese, French, etc. etc.)  You may need to contact Steam/Activision support on this one but as I said, at least attempt to verify the files, it may be that simple...