WSE backup via ISCSI fails


I’m using a DL4100 with 4x 6 TB drives in Raid 5 configuration. I made 2 ISCSI targets each 5TB in size.
So far everything worked fine until I planned to use one of the targets as a bare metal backup for a WSE 2012 R2 server using the Microsoft backup solution as it is integrated into the WSE environment.
Each time Backup will start and run for about 1 hour and then the backup fails ansd WSE reports an error with the drive advising to check the drive for errors. Checking the drive does not show any problem. This happens every time over and over again.