Physical machine backup fails with Error: -1 - An unknown error has occurred. (4041782)

We have noticed that the vRanger physical machine backups keep failing with Error: -1 - An unknown error has occurred.

[2018-01-30 14:40:09.619]: vRanger Backup & Replication - v7.6.4.0

[2018-01-30 14:40:09.620]: Selected Options: Enable Active Block Mapping™ (ABM). | Enable VSS application level quiescing | Selected SpaceSavingTech: None.

[2018-01-30 14:40:09.623]: Task for physical machine 192.168.0.28 was queued.

[2018-01-30 14:40:10.588]: Beginning backup task for 192.168.0.28

[2018-01-30 14:40:10.589]: Starting task validation.

[2018-01-30 14:40:10.632]: Connection to 192.168.0.28 was properly validated.

[2018-01-30 14:40:10.633]: 192.168.0.28 is properly licensed.

[2018-01-30 14:40:10.638]: Test connection to repository BK_DR starting...

[2018-01-30 14:40:12.121]: Test connection to repository BK_DR successful!

[2018-01-30 14:40:12.125]: Ending task validation... success!

[2018-01-30 14:40:12.126]: Beginning initialization of backup information.

[2018-01-30 14:40:12.127]: Retrieving the tasks parent information.

[2018-01-30 14:40:12.139]: Retrieving save points for any full backups associated with this job.

[2018-01-30 14:40:12.143]: Finished initialization of backup information successfully.

[2018-01-30 14:40:12.143]: Initialization was sucessful. Backup type to run: Full

[2018-01-30 14:40:12.149]: Gathering data for 192.168.0.28 from API completed.

[2018-01-30 14:40:28.320]: Creating snapshot completed.

[2018-01-30 14:40:29.921]: Generating Active Block Map™ completed.

[2018-01-30 14:40:33.833]: Removing snapshots completed.

[2018-01-30 14:40:34.083]: Error: -1 - An unknown error has occurred.