What I find bizarre is you can't connect to your own server using 127.0.0.1 which is the number for a local host, it shouldn't need to go through any routing for you to connect to your server... hmmm... pretty strange. So, let's start with looking at possible reasons why you wouldn't be able to connect to your own server.
You've tried with bullguard disabled, so unless it has active networking rules in place even when it's disabled we can hopefully ignore that as a confliction. All though you're welcome to attempt to uninstall it temporarily and try that way so we know for sure. For now let's assume it isn't this.
I wonder if Windows Firewall was turned on during one of the quality updates.
Let's try verifying that Windows Firewall is turned off.
If not, then let's make sure Windows Defender is off, if you have your own personal anti-virus installed it should have automatically turned itself off to avoid conflicting/competing antivirus software.
Let's make sure Windows Defender is set to off (you need only do the part of the tutorial that has you use the Windows Defender toggle buttons, ignore anything about group policy and registry edits.) and test to see whether you can join your own server.
If none of that allows you to connect to your own server let's do a test that verifies something. Unplug the Ethernet from the back of your machine so that it loses all internet, now attempt to connect to your own server using 127.0.0.1. If the connection succeeds something about your networking situation is causing the issue. If you are still unable to connect to your own server then something in your own system environment is causing the issue, because the exact same issue occurs on multiple machines we can start by identifying commonalities between the two systems.