(yup, it's still the author of the original post, but my other browser just remembers another set of credentials).

If I increase verbosity of the error_log, I'm seeing additional messages in log, like

upstream server temporarily disabled while reading response header from <backend IP>

but this message doesn't explain why the upstream server was disabled. I understand that the error occured, but what exaclty ? I'm used to see timeouts instead, or some other explicit problem. This looks totally mysterios for me. Could someone shine some light on it ?