MyBook DUO 2016 degraded raid 1

MyBook DUO, 2x4T disks, macOS, USB. About 2 months ago I started to get a dialog “Error in RAID configuration”, run WD utils, etc. Running WD Utils was not helpful, all checks were fine. Until two weeks ago, when RAID status showed up as “degraded”, and one drive as “FAILED”.

I replaced that disk, rebuilt RAID, which took 48 hours for 4T, and for a week all was fine.
Now I am getting the same dialog popping up, “Error in RAID config”, and again Drive 1, the new disk, as “Failed”.

I reformatted the old replaced disk in an external box, so far all checks are fine, fsck_hfs -fy -S (for finding bad sectors) runs fine, too.

Anyone recognising this behaviour? Might the enclosure itself be failing? Or am I simply unlucky and has this new disk failed already?

Bought that disk new, but if I register it on the WD site for warranty it says the warranty expired in 2016. What does that mean? Is it a refurbished disk? Or too old a disk to be in warranty?

Bought a new (non WD) enclosure capable of RAID 1, put in the original “FAILED” disk from the DUO, and the new “FAILED” disk, and everything now works fine. I suspect the DUO enclosure is failing, as both disks appear to be fine.

Well, this “solution” avoids losing data due to the DUO encryption, too. I was not aware of that, am now. A disk from the DUO indeed is unreadable.