For me the solution earlier mentioned by isaacery was to create a new VM with exact the same configuration as the one which does not work (with the primary vDisk location pointing to the correct old disk). The new VM booted instantly without any problems.
My problem started when I changed the Graphic card from VNC to the GPU (and back again as the VM did not start anymore). In VNC I got the message "Guest has not initialized the display (yet)".