Actually, I did think of one more thing. I don't know what your ISP is (or the hardware you have to connect to it), but if you have the ability to drop the DHCP lease and get a new IP address assigned to your router, I might try that just for kicks at this point. If that causes it to work, you definitely know it's something caused by the origin address. (And as a bonus, you would be able to play without VPN again!)

If it doesn't fix it, it doesn't give any new data, but it doesn't rule out past theories either; your new IP address might be in the same subnet as your old.

But at this point, anything else we try is mostly going to be just spitballing ideas and seeing what happens if we tweak any one variable in this connection.