"Read error" on long backups of Windows clients

Hello.

I've got some clients that take very long time to do a backup (mostly 
full, but also differential and incremental on some occasions).

The backups start normally, with the client generating a VSS snapshot.
However, after a variable amount of time (usually between 1 and 15 
hours) I get a "read error" on 1-2 files with "ERR=The system cannot 
find the file specified".
All subsequent files are printed with "Could not stat {filename} ERR=No 
error".

My hypotesis is that the VSS snapshot is gone.
Is this possible? Has anyone else seen this behaviour?
Does Bacula lock the snapshot it created in some way? (Can it?)
What could be the cause?

  bye & Thanks
	av.

P.S.
I read the FAQ that say I should investigate VSS with other tools; I'll 
eventually do, but if anyone knows an obvious answer, it would save me a 
lot of time...