virtualbox.org

Sorry to reply to myself. I should try the WDDM issue @hellAt said above. After disabling WDDM in group policy, the guest doesn't hog the CPU resource after disconnecting the RDP. Also the original RDP client works, too. (But I keep the new client since it could resize the window on-the-fly.) So it ...

I had similar problem with Windows10 1903 guest in July. Guest boots up just fine. First login with RDP to the guest works fine (to the guest, not VRDE.) After disconnecting, I can no longer RDP in from the same machine. Host (a 64bit Linux) shows the virtualbox process hogs all CPU resource I gave ...