We are also having this problem, in 2 old scene files we are working on. Reloading the settings as suggested does not work for us. Restart slaves is checked on the workstation that submits the job to backburner as well as on the DR host via the vray_dr.cfg file.
Strangely it seems 2 of our farm are immune from the problem, but the other 20 all hang and sit there using 1 core at 100% until the spawner is restarted manually.
Let me know if you need any info from me to help troubleshoot.
Cheers,
Olly