I didn't mention I'm running it in fullscreen mode, so there is no system bar. (In non-fullscreen mode, of course I can use the system to switch to tother app and desktop.)

Anyway, in normla or fullscreen mode, ctrl+backspace doesn't work, it's treated as key input and sent to the remote side.
Even after installing camkeyd, the focus key was sent to the remote side and I could hear my PC made some invalid key beep.

1.6 is required if the remote side is not using default rdp port, and it only has command line interface.