I've mentioned this in the Discord but didn't have all the details. I've done a bit of testing now and it looks like using the latest version of Plutonium prevents you from running multiple T4SP servers on the same machine. The second server window just suddenly closes and this is the last thing you see...
My ports and keys are set up fine. I'm using ports 4000 and 4200 on each machine, and I've tried other ports, including 28960.
When I rollback to an older version everything starts working again, and then updating back to the latest version breaks it again. I've tested it on my own PC too and the same thing happened. T5SP, T5MP, T6SP and IW5MP are unaffected and are all working as usual. I don't have any T4MP servers so I don't know about that.
I'm happy using the older version for now, just worried it will eventually stop accepting connections from the latest client.
Edit: I should add, all the machines I've tested it on are running Windows 10. And, although I have some campaign servers and that's not supported, I've tested it with multiple Zombies servers without campaign too, and got the same result.