500 Error with NGINX reverse proxy

[john-f-chamberlain]

When using this theme, the following error appears in my NGINX error logs:

2021/12/03 15:59:20 [warn] 4315#4315: *692657 upstream sent more data than specified in "Content-Length" header while reading upstream, client: xxx.xxx.xxx.xxx, server: mydomain.com, request: "GET /wiki/index.php?title=Main_Page&useskin=tweeki HTTP/2.0", upstream: "http://127.0.0.1:81/wiki/index.php?title=Main_Page&useskin=tweeki", host: "mydomain.com", referrer: "https://mydomain.com/wiki/Special:Preferences"

I'm using nginx as a reverse proxy to Apache with PHP-FPM and do not have any issues with themes other than this one. I'm running the latest version of MediaWiki (1.37)