I have experienced this exact problem when trying to use a text box in Nprinting. My work around was to create a formula in Nprinting that represents the value that you want to include in your report text box. Do not bring the Text box across from Qlikview. Just rebuild it in Nprinting template and include the formula.
Here is the formula I built in Nprinting to display in my text box ServiceRateLives. I created a variable in Qlikview so all I have to do is display the result of the formula. You can see the result of my formula is 3 Lives. I even wrote the formula so that it would change from life or lives based on the result.
I then create a new text box in my template and place my formula in the middle of the Text where I want it.
I am not sure why the formula is not maintained from the text box, but I did that sample preview error that you are showing.
Hope that helps!
** If you are coming to Qonnections in Orlando, come see my presentation on using Nprinting On Demand at 10:30 on Wednesday May 4th.