I have no idea why it's happening; the only oddity I see is that the username contains two spaces (as in "GHOST<space><space>king"), but I don't see how that would break anything.
It's not supposed to allow multiple consecutive whitespaces in names, so the reason it doesn't work is because of that. Most likely an account created before the forum was enforcing that properly. You can use the uploaduid qualifier instead.
Follow the link in the post you quoted, it gives an example. If it is how to find the user id, then you can go to their profile page by clicking on the mail icon next to their name in a gallery, or by searching here, https://forums.e-hentai.org/index.php?act=Members . Their profile page has the user id in the url.
When you type in "GHOST" in the "Member name begins with" field, it will list all members whose name contains "GHOST" in the beginning. You can then go through the list for the uploader you're looking for; in your case, this one: