@sirrick
I have the same problem. I have disable my firewall in windows, open my ports in the router, a static IP4 address. But if I start my masternode is the same error displayed:

"Not capable masternode: Can't detect valid external address. Please consider using the externalip configuration option if problem persists. Make sure to use IPv4 address only."

I do not understand why it should be a security problem to run the MN on the desktop at home. if you rent a server the coins somewhere in the world on a computer, since can also happen a lot ...