Strange ERR_CONNECTION_CLOSED for one specific site

[andre.lackmann]

I'm using OPNsense at home as the border gateway and largely it's working fine. I'm having one strange issue with a single site. If I try and access anything on the 'passport.online' domain in a web browser on multiple clients (only tested Mac, but on Chrome, Safari and Android Chrome) I get an error:

This site can't be reached
stratechery.passport.online unexpectedly closed the connection.

If I tether to my phone's cell network, the issue goes away and the websites work as expected. I'm not using any of the web filtering / outbound proxy functions in OPNsense (Services / Web Proxy is disabled).

I suspect a SSL / certification issue of some sort as the browser reports the web page (eg. https://stratechery.passport.online) is NOT secure. Again, if I simply switch to mobile tethering, same pages are secure, so root certs in Chrome or ay settings on my Mac don't appear to be causing the issue. If I try the same on my phone on the wifi going through OPNsense - it also fails. Suggests to me something in the gateway is causing a problem. The problem occurs whether I'm connected via WIFI or ethernet.

I'm a little stumped. Any suggestions are welcome.

[andre.lackmann]

Writing it down gets the 'ole noggin going.

So I also use ZenArmor and turning this into passthru mode confirmed that it was responsible for the blocking action. Looking at the settings further, I've added that domain to the Allowlist and it's now functioning. The reports show it was being flagged as a 'Potentially Dangerous Site'. Because I'm using the FREE version, there is no block message or template, so it's not clear when ZenArmor is doing this from the request.