Chris,
The Application Log doesn't seem much use as all I can see there is exactly the same error message that I originally posted. No other errors.
I would suggest running the following command (from CMD, run as admin) before and after each backup:
vssadmin list writers
This may give us some clues as to when the writer(s) are going into a failed state. At this point, we cannot be sure if this is a VSR issue or something specific to the VSS writers (part of the operating system).
A support case may be needed to investigate this properly.
