@matigo Thanks Jason. I'm trying to do some more systematic "debugging," if you can call it that, and it looks like I can get two kinds of erros when attempting to deploy the webhook. I can get both a 301 and a 404, and I think the 404 is a response to the htaccess.
I'm going to try and remove the block on php files in that folder now; just worried about the security implications. Not sure how many other PHP files there are in that folder, but it has to be worth a quick try.