OOM errors started when the mover ran, I had a similar one recently and from what I've read they are quite common with kernels 4.8 and 4.9, apparently fixed on kernel 4.10, but unRAID is still on 4.9.   Since then I decreased my RAM cache and so far no more OOM errors, you could try the same, manually or using the Tips and Tweaks plugin.   Values to change are vm.dirty_background_ratio and vm.dirty_ratio, default is 10 and 20, I set mine to 1 and 2 just to see if it would hel