WebService "Service Unavailable" error message

When changing the application pool user it is important to add the user to the IIS_WPG group.

Yesterday, we so this error message and found the solution here which says that the reason for this kind of error message may be because one of the following occurs:

1. The application pool is not running.
2. The application pool account uses an incorrect password.
3. The application pool account is not a member of both the IIS_WPG group and the STS_WPG group on the server.

In our case it was the third one, after adding the application pool user to the IIS_WPG group the WebService run like a magic.....