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From the Adsense FAQ I can only find a mention of PSA ads being displayed before my site gets spidered. Is this 403 normal, or should I start investigating?
where I assume [my-account] is my id and [my-url] is my page address.
When I paste this direct into a browser I get an error. If looked at other sites' code, and their url calls are very similar.
The main difference is their account id... the one I looked at seemed to have a named id rather than a string of digits... is this a clue?
When I pasted my ID into their URL, I get a 403 error again...
Go back into your AdSense account and get a fresh copy of the script, just leaving it at the default colors. Cut and paste it and see if it works then.
Secondly, was your application for AdSense on this site initally rejected, then accepted? Rejected URLs have a block placed on them, maybe they are using 403 errors now for them.
If the above doesn't resolve it, just drop an email to email@example.com explaining the problem. Leave the code up on one page, if possible, so they can take a look at it, too.
<added>I remember someone a couple of months ago mentioning having trouble using AdSense because his site wasn't yet in the Google index. Google told him to wait until it was in the index before using AdSense again. Not sure if this has changed since then. </added>