This is from the updated doc. 
http://seer.entsupport.symantec.com/docs/302438.htm


The backup fails with status 156

A backup failure with status 156, ?gsnapshot error encountered,?h can occur for 
several reasons:
?
You cannot run more than one backup per virtual machine at a time. If you start 
a second backup of the same virtual machine while the first backup is active, 
the second job fails with a status 156.
?
If the VMware backup proxy server has insufficient space in which to mount a 
snapshot of the virtual machine, the backup fails with status 156. 

The bpfis log on the VMware backup proxy server may contain a message such as 
the following:
13:35:37.859 [5536.1276] <2> onlfi_vfms_logf: [2007-09-21
13:35:34.578 'vcbMounter' 2204 error] Error: Failed to export
the disk: There is not enough space on the disk
Free up additional space on the proxy server and rerun the backup.
?
If the VMware backup proxy server does not have access to the VMware
datastore (VMware virtual disk files), the backup fails with status 156. The
bpfis log on the VMware backup proxy server may contain messages such as the 
following:
12:23:29.998 [3892.3332] <2> onlfi_vfms_logf: [2008-05-07
12:23:26.279 'App' 3668 trivia] Attempting to open
LVID:46a9ed36-ca235c12-1236-00001a1a24e4/46a9ed35-aa804fd4-6b99
-00001a1a24e4/1.
12:23:29.998 [3892.3332] <2> onlfi_vfms_logf: [2008-05-07
12:23:26.279 'App' 3668 error] No path to device
LVID:46a9ed36-ca235c12-1236-00001a1a24e4/46a9ed35-aa804fd4-6b99
-00001a1a24e4/1 found.
12:23:29.998 [3892.3332] <2> onlfi_vfms_logf: [2008-05-07
12:23:26.279 'BlockList' 3668 error]
12:23:29.998 [3892.3332] <2> onlfi_vfms_logf: [2008-05-07
12:23:26.279 'SOAP' 3668 trivia] Sending soap request to
[TCP:namur:443]: release
12:23:29.998 [3892.3332] <2> onlfi_vfms_logf: [2008-05-07
12:23:26.560 'BlockList' 3668 info] Closing connection
namur:443:root
12:23:29.998 [3892.3332] <2> onlfi_vfms_logf: [2008-05-07
12:23:26.560 'SOAP' 3668 trivia] Sending soap request to
[TCP:namur:443]: logout
12:23:29.998 [3892.3332] <2> onlfi_vfms_logf: [2008-05-07
12:23:26.638 'vcbMounter' 3668 error] Error: Failed to open the
disk: Cannot access a SAN/iSCSI LUN backing this virtual disk.
(Hint: Option "-m ndb" swtiches vcbMounter to network base disk
access if this is what you want.)
12:23:29.998 [3892.3332] <2> onlfi_vfms_logf: [2008-05-07
12:23:26.638 'vcbMounter' 3668 error] An error occurred,
cleaning up...
?
A disk was mounted on a virtual machine but the disk is assigned to the
virtual machine in raw mode. As a result, the attempt to create a snapshot of 
the virtual machine fails. The bpfis log on the VMware backup proxy server may 
contain a message such as the following:
12:46:27.175 [2636.5628] <2> onlfi_vfms_logf: [2007-10-17
12:46:27.019 'App'
1976 verbose] Fault Msg: "Virtual machine is configured to use a
device that prevents the snapshot operation: Device '' is a raw
disk, which is not supported."
?
Backups that use the 1-FullVM, 2-Mapped FullVM, or 3-FullVM and file for
incremental option are not supported if the snapshot mount point on the
VMware backup proxy server has compression or encryption enabled. For
the backup, the VMware backup proxy server copies the space-optimized
VMware virtual machine images to its snapshot mount point. The Windows
NTFS file system allocated for the snapshot mount point should not have
the Compression or Encryption attribute set. If either attribute is set,
backups succeed but single file restores cannot be performed from the
backups.

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