I don’t back up to nas so I can’t fix that specific error, but one excellent troubleshooting step you can do is attempt a restore from that volume. 

The requirements to restore are fewer than a complete backup. 

Next step is to add an external drive and add it as a second destination. See if Time Machine can do a backup to a different clean disk. [It will still continue to try to back up to the nas every other interval][1], but at least you will know the system itself is good and keep backups current while you suss out the cryptic error code. 

Lastly, even if you can’t restore from the sparse bundle, can you select it in finder and get it to mount? That will let you know if it needs repair in Disk Utility. 


  [1]: https://discussions.apple.com/thread/4987826