Uncorrectable errors on freshly setup btrfs RAID 1

I have Turris NAS bundle with two WD Red Plus 4TB drives that were originally in mdadm raid with ext4 and I want to move them to btrfs raid. When I have recreated the RAID with btrfs copied some data and then ran btrfs scrub some uncorrectable errors appeared. I have since then recreated the btrfs setup few times and when I have fresh setup and copy data on it, then immediately I get uncorrectable errors.

I ran extended smartctl tests on the disks but it did not discover anything wrong.

Would you have some suggestions on what could be wrong? Please?