VBoxVRDP -startvm "GuestOS" >~/VBoxVRDP.log 2>&1 &
inside the ssh session.
Whenever closed the ssh session, the virtual machine was killed. I thought it was a problem of the terminal, so I tried out screen and dtach. But the problem was remaining. Googling around, I finally found:
http://www.virtualbox.de/ticket/722In effect, I had shared clipboard enabled. After I disabled it, I could finally start VirtualBox using:
ssh -x firstname.lastname@example.org 'VBoxVRDP -startvm "GuestOS" >~/VBoxVRDP.log 2>&1 &'
Now, I'm a bit happier.