vRanger backup fails and the same error is present in the tasks and events tab of the virtual center for the given VM guest.
Datastore block size is not large enough to support disks getting snapshotted for backups.
For VMware versions prior to 5.0, by default, snapshots will reside on the datastore where the .VMX file resides, regardless of what datastore the disk .VMDK is on.
For VMware version 5.0 and newer, by default, the snapshots will reside on the same datastore as the root .vmdk file.
Move VM to a datastore that supports a larger file size.
More information here