remote desktop doesn't work on Hyper-v physical server

Hi,

there are two  PCs,  one is Hyper-v physical server,  one is workstation, 

there is one network adapter on hyper-v server , and i have create external virtual network adapter on it, at this time, i cannot hyper-v server from workstation through remote desktop connection ,  

if i remove that virtual network adapter , then i can connect to hyper-v server through remote desktop connection.

(then actual problem is that i can connect to hyper-v server, but the screen is black)

please advise where is the problem? 

any ideas will be appreciated. 

thank you