ReadyNAS104 - 8084 Error when copying files

I was copying a few jpg files across from my ReadyNAS104 to a folder on my Macbook using Finder.  The first two dragged/dropped ok but the third one gives a 8084 error message.  I've retryed the ones that worked and they now do the same.  Other images in other folders on my NAS copy ok and even the ones that failed can by downloaded ok via ReadyCLOUD.

I've rebooted my Mac and also updated the NAS firmware and rebooted that as well.  Still no luck.

Any help and guidance would be gratefully received.

Thanks

Peter