in the last 30 days, I had on two difference Qlik Sense Server, one was a customer server and the second our internal development server, the same error ERR_CONNECTION_REFUSED.
A short look in the logs I found this indicator for corrupted certificates "Starting proxy, waiting for certificates to be installed"
There are many things that can cause this, since it outside of Qlik this might be a question for Microsoft on why certificates are getting corrupted. (usually its something in the environment)