Live Merge fails with "Error invoking callback method doPolling for ACTIVE command"

Issue

• Live Merge (snapshot deletion) fails.

• The engine log contains;

2016-02-24 20:11:44,961 INFO  [org.ovirt.engine.core.bll.RemoveSnapshotSingleDiskLiveCommand] (pool-6-thread-1) [69b0f4a1] Running command: RemoveSnapshotSingleDiskLiveCommand internal: true. Entities affected :  ID: 00000000-0000-0000-0000-000000000000 Type: Storage
2016-02-24 20:11:47,094 INFO  [org.ovirt.engine.core.bll.RemoveSnapshotSingleDiskLiveCommand] (DefaultQuartzScheduler_Worker-75) [69b0f4a1] Executing Live Merge command step EXTEND
2016-02-24 20:11:47,107 ERROR [org.ovirt.engine.core.bll.tasks.CommandExecutor] (DefaultQuartzScheduler_Worker-75) [69b0f4a1] Error invoking callback method doPolling for ACTIVE command 15ab3c7e-c3ac-4e51-87ff-0f5df3d13f4b
2016-02-24 20:11:47,107 ERROR [org.ovirt.engine.core.bll.tasks.CommandExecutor] (DefaultQuartzScheduler_Worker-75) [69b0f4a1] java.lang.NullPointerException
2016-02-24 20:11:57,128 ERROR [org.ovirt.engine.core.bll.RemoveSnapshotSingleDiskLiveCommand] (DefaultQuartzScheduler_Worker-91) [69b0f4a1] Merging of snapshot 6e7de22d-c2fd-48c1-ac96-6ea1ce8c9f93 images be8df718-4c70-47cc-a046-6450b637d10a..588217c6-1048-4c3d-8ab6-15584b60dab4 failed. Images have been marked illegal and can no longer be previewed or reverted to. Please retry Live Merge on the snapshot to complete the operation.

• The Event Log in the RHEV Admin Portal shows the following generic error;

 Snapshot 'snapshot-x' deletion for VM 'vm-x' was initiated by admin@internal.
 Failed to delete snapshot 'snapshot-x' for VM 'vm-x'.