I would recommend upgrading your Qlik Sense Server to 3.1, as there are fixes going on in both Sense and NPrinting that work together.
Have you tried manually updating the cache on the connection, using the cog on the connections page:
It shows you for each connection you have when the cache was last updated, so you can see if that is before or after your app change.
Hope that helps.
yes, I consider to uprade my Sense Server installation as you suggested.
And yes, I manually update the cache every time I made changes. That's what I tried to say in my initial post with "reload metadata".
I recently discovered a workaround to force the reload. Therefore I restart the NPrinting Engine Service (in Windows Services). After this restart, I have to wait several minutes for NPrinting reconnect to the Sense Engine, as shown in "Engine manager -> Status Online". As soons as the engine connection is up again, I update the cache for my connection manually. This works! All changes are now recognized by NPrinting and can be used in Template Editor.
The Qlik Support recently confirmed, that this behavior is a bug. It should get fixed in NPrinting 17.3.
The support also suggested a service restart as a workaround.