I’ve seen this before if one of the required PHP extensions is missing, perhaps that’s the issue. If you click Data Admin > Data Admin Settings are there any red X’s next to the system requirements? Otherwise, be sure to check your PHP error log to see if there’s any recent messages in there that may be related.
Okay, the next step would be to check your PHP error logs for any clues.
Also, does the export function work for other areas of Gibbon (eg: Query Builder or Markbook). This may help indicate if it’s an issue with the configuration in general, or just that one script.