Check to make sure you are not using any of these characters (  / : * ? ” < > | ) in the index field because these are invalid filename characters for Windows. Windows will not allow you to create a file or folder that has any of these characters in the filename. If you strip out the invalid filename character from the index field that you are using to name the tiff files and re-export the data group, the export should run fine. (Also ensure there are not any blank values in that index field. )