Windows backup not using 2nd destination disk

Hi,

I have quite fresh Windows Server 2012R2 setup as the only DC in domain, having some shares and that's more or less all about server roles. As it has 3 RAID arrays (RAID1, RAID 10 and RAID 5) the latest (RAID5) is used as LOCAL BACKUP destination (built-in Windows backup is used). Backup is running fine, no problems.
Additionally we bought QNAP NAS and configured it as iSCSI target to serve as secondary backup destination.

But Windows Server simply IGNORES that 2nd backup destionation on iSCSI Target. No errors, no event logs...nothing....it just does not use it. Showing each backup success, but iSCSI location is empty, zero backups, nothing there.
...while primary location on local RAID5 array backups just fine.

What I have tried:
- reformated QNAP NAS to factory defaults and setup iSCSI Target fresh
- used it as no-drive-letter and as drive-letter volume
- used it as external disk and as volume
- cleared backup schedule and recreated it fresh
But nothing helped - iSCSI target destination is writeable, visible, but simply ingnored as 2nd backup destination.

Any idea?