The subnet in question is 10.2.4.0/24 this is where the additional bridge is being added, it's also where my computer is. The main Proxmox bridge is on subnet 10.4.4.0/24. I didn't find anything being blocked in the logs.
Very interesting! Looks like the same issue to me, ping and tracert look similar too. Hopefully someone will chime in who might know a fix, this is over my head.
@MarioG is the machine you are using to access Proxmox on a different subnet? I feel I'm having a very similar issue to yours. I can access the gui if my machine is on the same subnet, but as soon as I move to the other I loose everything, gui...