When I try to export Student Data as a LabChart file, I just get a blank window, and no option to save the file. Why is this?

When a student has started an experiment, the option to Export as a LabChart file becomes active - you can locate the export option by searching for the experiment in the LabTutor Server under the 'Find and Manage' sub-tab of the 'Student Data' tab.
This export is available regardless of whether the student has recorded any data (or added any example data) or not. 

If you try to export and no option to save the file appears, then the student / group has no data within that Experiment at present. As soon as the student or group records or adds data, then Exporting will allow a LabChart file to be saved.