One likely cause could be that the code within the questions.js file is broken because of some characters that you have added to the data tab. Our routine attempts to address all possibe issues, but it is still possible that this could arise. We suggest that you delete a few lines to a hundred lines at a time and try to identify the question that is breaking the file. If you use a color-formatted editor such as editplus, it will also show you where the possible error could be - by a change in the text color.
