I found the following additional information which also seems to relate to the above issue (in the Google Help files) -
Quote
Why am I getting PSAs?
....
Your web pages are behind a login.
At this time, pages that require a login can not be easily visited by our crawlers. Additionally, since these types of pages are not easily visited for approval and it is difficult for our AdSense specialists to verify that pages behind a log in follow Google AdSense policies, we are unable to offer support for these pages.
It seems that Adsense ads in a private forum might still appear but it is harder for Google to work out what ads to serve. Therefore, you will have a higher likelihood of having Public Service Ads being displayed. And these of course do not provide you with any revenue.
I had another look at the private forum on one of my other sites and about half the ads are off topic (relating to forum software and servers etc) and the other half are somewhat relevant but not specifically relevant to the given pages on which they appear. They match the theme of the site but not the words on the individual pages in the prviate forum.
Let me know if you find the same,
Cheers, Glen