It's a very long-standing issue, I'm actually quite surprised this is the first report here on GitHub, or how nobody's ever took the time to fix it.

I took a look ages ago, I think the problem was something along the lines of re621 providing the IQDB endpoint with a local file path instead of uploading the file, which obviously won't work, resulting in an error.

It's worth noting that it functions perfectly fine when using URL uploading, which is generally recommended for many other reasons too.

This may or not be an issue because I never sat down and tried to figure out how to pass on the local file over to IQDB.
I wish I had more time to work on re621 these days. I do realize that this should be addressed at some point, but it's unlikely to happen soon.

I wrote a fix, but there are some limitations due to the nature of querying with a file, since it requires using a POST request instead of a GET request.

I couldn't add a "Duplicates Found" link that linked to /iqdb_queries and passed the file like the URL search does, the dupes only show up on the upload page.

Likewise, I couldn't add any additional reverse image search links in risContainer with file uploads. Image query POST requests would all likely need an API key.

Finally, I couldn't figure out why the handleInput function doesn't fire on drag-and-drop. I tried adding the drop event to the listener alongside input and paste, but nothing happened on drag/drop.

Ultimately, those issues are minimal or beyond the scope of the weird 403 error, which has been resolved.

Considering that I was only expecting, at most, for the check to be disabled when posting a local upload so the error wouldn't show, even a partially working or roundabout way to get an actual dupe check done would be above and beyond. Thanks for your hard work!