I open up my project in Nprint, drop and drag a text box to Nprint - where it displays. I save my project and exit Nprint.
Next day when I come in and I select Edit Template (Powerpoint) i get that message.
it either loses connection to the object, or I suspect the security on the model messes with Nprint....
I have been able to sccussfully crate PowerPoint reports - there have been several customers running into issues that sound similar or exactly the same as your issue.
One option to try to see if the file is really correupted and at what point is after you create your tempalte, save it, exit Nprinting and go into PowerPoint and try and open the PoertPoint tempalte