I presume it's to late for you but maybe it will help others:
After i defined a new connection i had that same error. On all the report that was already built on that application.
I suspect that NPrint doesn't support 2 connection type on the same application. Until today all the connection was as Type: Server and today i defined a new connection as LocalFile.
In this moment the error appeared.
After i deleted that connection and recreated as Type:Server it started working again....