If you are using Qlik Sense 2019 Feb release, note that there is a bug in that QS version affecting the included REST connector component which is causing NPrinting API connectivity issues. An update is expected in the June release of Qlik Sense but hopefully sooner. A fix was already introduced for QV users. See https://support.qlik.com/articles/000059611 for more information.
If you are not using QlikSense Feb 2019, then it is more likely a problem with your API syntax. (the bug mentioned above is not with NPrinting API's so it cannot be repaired by upgrading NPrinting. However its good that you are using April 2019 to leverage other feature enhancements. See product release notes with that come with that version)
However, to be sure it is the same issue caused by the rest connector with Qlik Sense Feb. 2019, I suggest that you submit a support request in order to investigate more closely.
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