You can't really tell a lot from that 1 line. Usually arond that line I'm used to seeing 'cant find xxxx as a vm, can't find xxxx as a datastore, can't find xxx as a ....", which usually means something like an object was deleted from vCenter but is still part of the backup job or something of that nature.
We have the same problem. In our case, I believe the problem was a result of an upgrade from VSC 2.x to VSC 4.1. We are seeing the exact same thing as smora_netapp and Tatsurou Ide. Same exact thing. Any one had any luck in resolving this?