OK, you have a backup system that works but it appears that you have a problem folder on your PC that is preventing one backup set completing (it may be down to just one file in that problem folder).
You should be able to find the problem folder because that is either an incomplete folder in the set on your NAS or, if there is no incomplete folder it is the next one that would have been written.
Once you know the folder or file check if it is one that conflicts with the cut down Linux OS that runs the NAS. If that is the case rename it to prevent conflicts and see if the backup completes. If it does, problem solved. If it doesn't post a full directory listing of the problem backup set and we mighr be able to spot the problem.