I had a weird issue registering hosts similar to this. It turns out the students had removed the belt from the dvd-rom drives and Fog would freeze for about 45 seconds. I guess it was trying to get the status of the device, but without being able to confirm if the drive was open or closed, it just sat there and spun the motor back and forth for a while.
I found ejecting the drive with a paper clip immediately caused Fog to continue to the normal registration.
Something else to consider is that the error you see has nothing to do with what is really causing registration to fail. Consider booting the debug mode see if the error you see is truly what’s wrong. There is a wiki guide to debugging somewhere.