Error reading from remote server

[#1]

While opening a page in classic UI we are getting this error it states that "The proxy server received an invalid response from an upstream server". We tried to increase the SSL timeout but still, error is not resolved.

[#2 · Accepted solution]

Do you have an SSL enabled? Is it possible that while you are requesting page via https, it's unexpectedly redirected to an http?

Another possible scenario was discussed here:

https://experienceleaguecommunities.adobe.com/t5/adobe-experience-manager/the-proxy-server-received-...