Yes, I know it's not supported, yet there's a category with hundreds of posts on the forum, so people are still doing it.
It's most likely a memory failure, the reason I have the second NAS is I bought it to copy over everything from this failing NAS with other discs, as I suspected the resync failure was caused by bad disks. It was stable with the disk out, and I copied everything over and bought new disks for this failing NAS, but now I have the exact same problem with new disks. The old disks are running fine in another computer now.
So my question remains; if I want to replace my memory, can I use ECC or not?