Vista backup shadow copy error 0x8004230F

I have Vista's automatic backup feature set to do an incremental backup
each evening at 9 pm, when I am not usually using the machine. However,
about one time in 3 it fails with an "unexpected error" as above, in the
"creating shadow copy" stage.
I am aware that a re-boot will fix this and allow backup to run.
However, I already shut down my computer every night so am unwilling to
do an additional restart each evening shortly before the backup is due
to run.
Is there any way to resolve or prevent this error without restarting
the computer?
I've tried re-starting the VSS service after the failure and then
re-trying the backup, but it always fails with the same error and will
not run again until the machine has been restarted.