Last Saturday/Sunday I had the problems repeatedly only on one schedule type, our weekly fulls, on several different policies. This weekend I had the problem only on our exchange server policy, starting at 12 midnight. The daily policy is the same, only different in name and calendar schedule - 1st through 4th sat/sun.
Please keep us updated if you find out anything new, this issue ate up my tape drives at inopportune times this weekend for sure.