It's possible that the data has changed.
Please refresh your metadata then restart your Qlik NPrinting services.
If your sense app id or qvw path has changed you should address that before refreshing.
If the data doesn't refresh in your template editor, try creating a new app, connection and report and retest.
Data refresh issues are going to be addressed in the upcoming release of NP 17.3 scheduled for release in Early February.
If you continue to experience the issue, please submit a support case at the Qlik Support portal with detailed steps to reproduce the issue.
Hope this helps in the interim.
This new error indicates that you may be attempting to export a pivot table.
I don't know what version of NPrinting you are running but this is fixed in NP 17.3 which will be released later next week or early the following week.
If you wish, you may also submit a support request and someone from our team will get back to you to investigate more closely with you.