I faced this twice . Im not sure why you are getting an error but It may help
Do you have any filters applied?
If yes make sure you apply filters according to priority and rules.
1. when I used nprinting for first time I had issue while Im applying the same filter to task, report and user at once but after looking in to below link I learnt that its wrong. please check this in your case too.
In another case please let us know if you could successfully edit the template , preview it and save it?
2. faced this second time while I executed the task which has a report for which the preview was not working(throwing an error because of one object). I hit run now for this report it mailed me an error which you were talking but not the attachment.