Just in case somebody is deperately trying to fix their problem just to realize there is nothing wrong with their reverse proxy setup:
In my case, the error persisted even after I’ve removed all
location directives but a single one that only provides static content.
The error message was caused because Nginx wasn’t able to log it’s log to the syslog server:
access_log syslog:server=10.0.1.48:514,facility=local4,tag=nginx,severity=debug,nohostname main;
If you are using a syslog log server, make sure it is available. To test whether the error originates from the logging setup, comment out all logging configs so that Nginx falls back to the native logging scheme.
I hope this saves some people time debugging a fully valid reverse proxy config, just to fid the error somewhere else 😀