When I go to /manage, there is no warning. When I click "ENABLE AUTO REFRESH" (or "disable .." if it was on already), the page suddenly shows a big red warning "It appears that your reverse proxy set up is broken."

I haven't set up any reverse proxy, so this seems wrong. I think Jenkins is confused by the parameter in the url formed: /manage?auto_refresh=false. Using the network trace in firefox shows the response header's location is set to:

http://XXX:8080/administrativeMonitor/hudson.diagnosis.ReverseProxySetupMonitor/testForReverseProxySetup/http%3A%2F%2FXXX%3A8080%2Fmanage%3Fauto_refresh=false/

for a request to:

http://XXX:8080/administrativeMonitor/hudson.diagnosis.ReverseProxySetupMonitor/test

which had a referrer of

http://XXX:8080/manage?auto_refresh=false

(I cannot determine from the description if JENKINS-23280 has the same cause, if so, it wasn't detected or described there.)