I don't know if I had the same problem as Javier,
I had to hold SHIFT and click the VATSIM logo to get it going as the initial error message suggested..
Mike's solution did the trick for me!
Thank you Mike!
I also have a "Forbidden" error when trying to get METAR of any airport.
This one started like 6 months ago..
You wouldn't know how to fix that one too by any chance Mike, would you?