martinibuster - 7:50 pm on Jan 17, 2011 (gmt 0)
It works for me too.
Did you just install those ad units to your web pages? If so then that's normal. I have seen that for code that has just been placed on a web page. It's a temporary error that lasts a few minutes to a few hours. Google's systems haven't updated the new ad unit and so it temporarily does not show ads for those web pages because it thinks they do not exist in it's systems. Hence the 403 error code:
The server understood the request, but is refusing to fulfill it.