I am having the same problem. I discovered that WHS2011 is creating files in the Temp folder to create the back-up files of the server before they are written to the server's back-up drive, tape etc.
Unfortunately it seems to not have the ability to automatically do any house-keeping of the temp folder.
I found that if you delete the contents of the temp folder prematurely (files created on the same day of a back-up) it will cause the server to possibly crash and then restart. It will somehow also delete your client setting, which means computers which are registered to be backed up regularly on your network become de-registered.
