From the looks of the error message it seems to be an error / warning reported on the backup application.
Does not seem like a VMware related error. ( does this happen only for VMs that are too big in size ? )
You can try the below -
Solution 1 -
Log file left in the directory. Clear it out, Reboot the VM, and kick off a backup.
Solution 2 -
perform storage migration elsewhere (to a different datastore) and then back to the designated datastore. This will make a cleanup of all file names – it will put all your vmdks in one folder and label them properly.
Source - http://jakubsojka.com/?p=145
In addition to the above will also suggest opening a case with the backup vendor support.