I'm getting an error "ASP_0131 Disallowed_Parent_Path" trying to include an asp page

You need to enable parent paths in the web server settings to fix this problem.

I ran into this on a HostGator server (shared) - and did the following:

Websites & Domains -> (Advanced) Virtual Directories

"Manage" the virtual directory in question, then "Directory Properties"  and check the "Allow to use parent paths" box.

That should fix the error.