"cannot parse response" (NSURLErrorDomain:-1017)

I ran across this issue and 329 people had the same problem back in 2016 (https://discussions.apple.com/thread/7430493), although I experienced the issue today 5 years later. The question was stale, so I couldn't post a reply with my findings which is why I'm asking the same question and answering it at the same time.

All of a sudden I started getting errors on all my Apple devices when accessing our website after installing an nginx-proxy in front of an Apache2 server. Firefox on my MacBook worked fine, but Safari on the same MacBook reported an error. I'm not that knowledgeable about HTTP2 and reverse-proxies so can't really comment on the root cause, but I was suspicious of a warning about HTTP2 in the server log. When I disabled (or rather didn't enable) HTTP2 on ap2 server the problem disappeared. I hope this helps other folks experiencing this problem