You have an extra snapshot file. I would first confirm that the vmdk of the failed backup is not attached to the VDP proxy. Remove it if it still is attached. Run a snapshot consolidation. I've found that rarely you have to do a storage vmotion when consolidation doesn't remove the extra snapshot files.
The key entry that is telling you what the problem is:
2015-09-01T20:00:08.647-02:00 avvcbimage Warning <18652>: Too many extra snapshot files (1) were found on this Windows VMs datastore. This can cause a problem for the backup or restore.
I believe there is a good chance this issue will be less common after the next major VDP release. Automatically cleaning leftover hotadd mounts is a new feature of VDDK 6.0. Avamar 7.2 is taking advantage of the new feature.