Bug XL/NC ver 3.6.15:

[ClydeL]

Opening and viewing a sequence in the Preview tab works the first time. After viewing, when adding another effect to any model, clicking SAVE, and going back to the preview tab, the preview will not play. I must exit from XL/NC and re-start.

Also, when first opening XL/NC, in the Preview tab, the "play" button shows as a "pause" button, but works correctly.

[Gilrock]

I saw similar thing with the Preview tab on version 3.6.14 but I was always able to just hit Open and re-open the file to get it to play.  Did you try that?  I never had to close the program.