>Has anyone experienced this??

Your first impulse should be to check the List archives, where prior
experiences are preserved.  There were a handful of prior reports on
this, but no definitive conclusion posted.  I'd suggest that you
review your server Activity Log for what else was running at the time.
I would expect something which is seriously updating the database,
such as an Expire Inventory, Delete Volume, or the like.  That's
where locks originate.  Review your admin schedules to look for
things which are best run away from backup windows.

  Richard Sims, BU   http://people.bu.edu/rbs