Got the same.
For everyone: GOT THE SOLUTION !
Go to your Mw2-folder, for me its:
C:\Program Files (x86)\Activision\Call Of Duty - Modern Warfare 2
And search for a file called alterIWnet.ini. Open it in Editor and add the following by replacing everything:
[Configuration]
Server=server.alteriw.net
WebHost=auto
Nickname=Add your nickname here!
[Dynamic]
ServerWarningShown=True
HackWarningShown=True
Own idea, worked for me.
Dont forget the thanks-button.
Greetz