MediaBox does not search all available (ones being up) net interfaces for UPnP services. It binds only to the first available interface...

For example: If you are connected to a network via wifi and it has some UPnP service, and next activate an OpenVPN connection with a network that has some UPnP service too, Mediabox doesn't see the services on the VPN network but only sees those on the wifi network.
The same network configuration works OK with the standard "Media player" which lists all the UPnP services on both networks (i guess it lists all services on all available interfaces).
Probably it will be nice to implement a loop for checking all interfaces so if the MediaBox player is running while an interface is added, it to be able to add the available UPnP services without being restarted.