Odd problem with running Nonlinear Curve Fit VI with two while loops

In the attachment you can find a very simplified version of the program I develop at the moment with LabVIEW Version 13.0f2.

I use the “Nonlinear Curve Fit VI” to fit a function to some data in my main while loop. The values of the fit function are then used in an inner while loop for further calculations. The fit works well, but if I want to stop the program, I have to press the stop button two times.

If I replace the “Nonlinear Curve Fit VI” by another VI (linear fit or similar) or if I put the “Diagram Disable Structure” to remove the “Nonlinear Curve Fit VI”, the program works as expected: It stops after pressing the stop button one times.

I do not understand this behavior. Is this a bug? Or do I oversee some simple mistake?

I would be very happy about every help!