Ok, thanks.

It's currently appearing on torrents that are still on the site. But I'll keep checking on it and see if it resolves.

I'm not having that problem at all today or  yesterday, so you might want to ask a mod if there could be another issue by using the Helpdesk in the menu under Support at the top of the page.

It's currently appearing on torrents that are still on the site. But I'll keep checking on it and see if it resolves.

I've been struggling to upload/seed torrents enough to maintain a good ratio and although transmission says it is seeding my torrents, the inspector shows an error message saying "Tracker gave HTTP response code: 404 (Not Found)".

Can anyone explain how to fix this and whether or not this is a blocker to other users being able to download from me?

Yes, that will prevent anyone from downloading the torrent from you.

Sometimes there are problems with the tracker and those error messages appear on many or all torrents and the problems will eventually resolve.  However, if a particular torrent is removed from the site, that message will be permanent and there's no point in continuing to seed it.