Hi Marc,
This is a known issue, moving a blog to a different page is not supported and best avoided. You can rename the page or move the page to a different place in the site hierarchy but you should not move the blog to a different page after it has posts and a blog instance cannot be used on more than one page.
The reason is because the friendly urls for blog posts map to real urls in the form /Blog/BlogView.aspx?pageid=x&mid=y&ItemID=z
and security is enforced based on the pageid and moduleid in the url to ensure they match the current page and that the current page contains the module.
The only solution other than moving the blog back to its original page which is what I recommend is to manually fix all the friendly url mappings for the blog posts so that the real url has the new pageid. This could be done under Administration > Advanced Tools > Url Manager but it would be very tedious if you have lots of posts.
Hope that helps,
Joe