looks interesting, especially since the Argument value for Web Requests should usually show the URL.
could you please right-click -> Details... on both of these doFilter() nodes and show us the screenshots of this.
ok, so based on this I would say:
bottom line: to me it looks like this AjaxFilter gets some sort of "null" string as value for the URL.
that's a good question. I tried to reproduce it, but I could not reproduce the exact same behavior.
if you want, you can send me a session with both those PurePaths and I can try to find out more details.