no live upstreams and NO previous error

[drookie]

(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 ?

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,269577,269583#msg-269583