When using the w3c validation service, you will receive about 50 errors for the startpage.
A convenient way to use this service is by using Opera were you can access it using a right click in the current page of the browser.
Thanks for your bug report, and the problem seams to be the component LogGenerator that not is XHTML compatible. The bug will be fixed to the 4.51 release. If you don’t are planning to use the Statistics it is possible to remove that component from DefaultFrameWork.ascx.