mdadm: Raid1 array constantly degrading.

For several weeks my Raid1 array is constantly degrading on Tumbleweed. Running two HDDs, one of them is suddenly inactive. Re-adding is no problem. What I did so far: - Backup as usual an hoping that things change due to some kind of regression being sorted out via update. - Checking several logs with help of the forum. I could not find a single hint of what is going on - even with running "dd if=/dev/sdb of=/dev/null bs=4M" on both HDDs. - Changing SATA cables. Please find a more detailed description on what has been done via this link: https://forums.opensuse.org/showthread.php/577384-Raid1-array-constantly-deg... Would highly appreciate any suggestions how to sort this out.