It was actually an NGINX issue - I had the same issue before and the nginx.conf had reset after the upgrade. Thanks to the post here: Nginx configuration - #7 by Arnaud
For anyone using plesk with an NGINX proxy, simply add the following to the nginx additional directives for the domain/site:
proxy_set_header Connection '';
proxy_http_version 1.1;
chunked_transfer_encoding off;
proxy_buffering off;
proxy_cache off;