The problem you report is quite common in NPrinting 15 and it is due to the instability of the license system used by that version. A minor change on the NPrinting machine or an OS update may cause the deactivation of the license.
The best solution is to upgrade to NPrinting 16 and activate the product with the new license code that Qlik porvided to you.
This will completely resolve the problem.
Please, contact the Qlik support if you can't upgrade immediately or if you cannot find your NPrinting 16 license code and control number. Be sure you specify the NPrinting 15 license code you are using.
Kindly, also notice that NPrinting 15 is not currently supported.