I found this error occurs if you delete levels files within a level pack you were playing (with the player open), close the player then try play testing (with editor) again later (on a level other than the one you deleted).
Removing userdata file works too, but also moving (not deleting entirely is necessary) the entire levelpack in question also works). Also apparently downloading a new NL player and changing nothing else also fixes it.
Should this topic be merged with Simon's?