So you're running ufsdump without fssnap to snapshot first?

Perhaps it's coming across what it thinks are open/locked files.

You should run fssnap first and then dump the special (static) device fssnap gives you.

Also, remember to delete the snapshot (again using fssnap) once the ufsdump has completed.

Failing that I'd be tempted to fully check the filesystem with

# fsck -o full

option. It will take a long time but will check everything on the filesystem. Of course, yes, it might be quicker to resort to tar but will that give you a faithfull restore of the filesystem?

Search this forum for threads about ufsdump AND fssnap. I've written about all this before on this forum.