When you access a virtual directory, you receive a 403 error (access denied error) even though the content does not exist. The GET request should return a 404 error (not found error).
The 403 error is supposed to mean you are denied access, which would imply that you need to log in, sign up, join up, or some other twat to gain access.
There are circumstances where a 403 error will be returned when the "correct" message should have been a 404 - page not found. One such circumstance is when you put an extraneous "/" or "\" on the end of an address. This is sometimes translated by the server as an html instruction, and it thinks you're looking for the "null" directory, which may or may not exist. Since allowing access to the "null" (or nul) folder is unusual, you may get a "you can't go there."
Note that the "/" and/or "\" characters may be legitimate in some addresses(?) but in cases where they're not, they may cause this sort of error. A similar extraneous character in the page script can cause the same, or similar errors. There's not much you can do about bad script on a posted page.
You can check the URL you're using, and make sure you don't have any extra end characters. If whacking of an end char doesn't help, then you probably need to register to get authorized to see the stuff you're looking for.