I was trying to upload an image from Gel/Dan/etc. Booru marked Safe because, despite my best searching, I couldn't find it on Safebooru (I used the multisite search, and it didn't find it either) but when I uploaded it, it told me that Safebooru already had it at this location:

safebooru.org/index.php?page=post&s=view&id=

Which, uhm. It doesn't. It doesn't have anything at ID nothing. Just... Kinda loads the front page. Anyone have any idea what's up with that and how to fix it?

It is not fixable. It is an empty space because it has been purged from the system Data Base at some point, for whatever reason.
Send a link by note/private message to an admin or mod to make sure that it is okay to upload. Take another look at your content and be sure that it is SFW and complies with the Terms of Service.