This one is now a classical
One of the related thread is here:
To sum up:
*** mösti explanation and workaround ***
When you connect to a remote windows using vnc you connect to te real screen. If you than also make a connection using RDP the real screen is blanked as you see in vnc and an windows uses an other driver for sending the screen trough rdp.
If you reconnect vnc you also connect to the "new" driver windows had loaded and you see the same screen as on your rdp session.
Then you disconnect your rdp session windows returns to the driver of the real screen and your vnc session shows a black screen because the rdp session driver where it is still connected to really gives a black screen because it isn't used anymore.
You have the same with switch user, only the user who logins first in windows has control over the real screen. The others are virtual.
But reconnecting vnc always works and solves the driver switch because it then reconnects to the display driver too.
*** Andrei workaround ***
You can still run both Remote Desktop and Ultra VNC at the same time.
Just connect to your computer from 2 different IP addresses: with Remote Desktop from one IP and with VNC from a different IP. (In most cases, this entails that you should do this from 2 different computers)
Thanks for their contribution