I'm not sure if I remember, but the correct syntax for a UNL may have changed a little sometime in the last year or two. Here's an example that works for me (it opens the outline in a new tab and goes to the specified location):
unl://c:/users/tom/.leo/utility.leo#Python-->Case insensitive Compare
You need exactly two slashes after the string "unl:". I know that a correct file: URL requires three slashes, but the UNL code wants two for reasons I don't remember if I ever knew. I'm not aware of any recent changes in this behavior.
If this is the problem, it can probably be fixed with a global search-and-replace. Otherwise, please post examples.