Initially this problem was occurring in a page with an enclosure and so I thought it was specific to enclosures but on minimalizing the quickstart it is apparent that it happens on any page where a non default namespace is specified AND a href has a relative path.
The attached quickstart has a single page with no components at all. In 6.x the page fails if a non default namespace is specified in the <html> tag. This works fine in 1.4.x and 1.5.x
Last cause: Unable to find component with id 'foobar_relative_path_prefix_' in [HtmlHeaderContainer [Component id = _header_0]]
Found with similar names: ''
If we remove the non standard namespace declaration from the html element then the page works fine.
Also, if we remove the line:
<link rel="stylesheet" href="style.css" type="text/css" media="screen" title="Stylesheet"/>
from the HomePage.html then the problem doesn't occur.
Problem could possibly be related to how RelativePathPrefixHanlder deals with a non default namespace.