hi @clempat, the internal bluetooth adapter is not serial port based device (unlike BlueGiga devices). You will have to use something different rather than ser2net, that allows you to create a virtual usb port over ip.
@Dibbler42, has created a manual how to do so: Forwarding of serial and USB ports over the network to openHAB - #3 by Dibbler42
If you had a BluGiga adapter, then you would be able to use ser2net.