We had a customer that was using a VPN / Firewall to access our cloud instances of Spira. Most firewalls simply pass through the HTTPS traffic on port 443 unaltered. However some more stringent ones actually decrypt the HTTPS traffic, inspect the packets and then re-encrypt it for the user. In one case, this was causing an ERR_CONNECTION_CLOSED error.
As you move your web applications (such as SpiraTest, SpiraTeam, SpiraPlan, or KronoDesk) to newer encryption protocols such as Transport Layer Security (TLS) 1.2, you will need to make sure the client applications are configured to use the latest version of TLS. Specifically applications using Microsoft .NET may not use the latest version by default.