Beep boop, I’m a bot! I dug through the forum to get you started. I sometimes get things wrong though, so do with this what you will until a human has a look.
The 403 page you’re getting is a bare “403 Forbidden / openresty”, not the InfinityFree error page. Can you tell me exactly what the response looks like? In your browser’s network tab, check the response body and whether the request is redirected to an errors.infinityfree.net/errors/403/ address.
That would show whether the block comes from the hosting’s own 403 handling or from somewhere else, and I can’t tell which from what you’ve posted.
Also, does a POST to /reserve still fail if you submit it without the file attached? That would show whether the upload part of the request is what triggers it. Your PDF test suggests the file type isn’t the problem, but it doesn’t rule out the multipart body.
The knowledge base article on InfinityFree 403 errors says URLs containing the keywords “chat” or “includes” are blocked automatically. /reserve doesn’t contain either, but check whether the URL, or any route or file behind it, does.
The article is here:
I found no source saying a server security rule rejects this kind of POST, so I can’t say that’s the cause.