I have encountered this problem a couple of times. Everytime this happens(in my case) it is because of the actual dialog box memory for studio spline going wild.
The way i have solved it is by going to preferences - user interface - Click Reset Dialog Memory - Restart NX.
This might not work for you, but a suggestion.
If this works for others or someone has another solution, please reply here for the benefit of other users.
When it happens the next time,
immediately after the error message go Help- NX logfile. Remove all text but ~the last two pages, upload the last two pages here and we can hopefully give you a hint of whats wrong.