I can't see Adsense (ad or link unit) ads here. You cannot put ads on an error page. Especially not on blank error page.
Adsense must be on pages with relevant content. You can however, use the 404 page to recommend sites that maybe the users were looking for.
putting adsense on 404 page is against adsense TOS. however, if you have lot of visitors ending up in 404 page, then you can name your 404 page something else like thankyou.html or whatever and put some content on it. or yuo can redirect your 404 page traffic to your about us page and put adsense there. just a workaround
i have the same template for my 404 page as my website, only thing being with an error message with 404! so yes my 404 does have adsense.
I have seen adsense ads on 404 pages and it is with in the TOS. But it doesn't make sense to place adsense ads on 404 pages, which may not be having any relevant content and the chance of click is very slim