I had a similar issue and I got over it. In my case I encountered a similar error Error contacting the Parsoid/RESTBase server: http-bad-status. I can tell that it's definitely the issue of rewrite(Nginx users probably, like me). But I had another issue as well, I had restricted the access via the Cloudflare's firewall by allowing only one IP address to have access for development purpose of course.
What I did was I included this in my Nginx config to make sure rest.php works even with a trailing slash:

location /wiki/rest.php/ {
    try_files $uri $uri/ /wiki/rest.php?$query_string;
}

Also, I had installed in a different directory rather than the main so be sure to include that directory as well just like what I did.