When running a site export the progress bar shows the export is in progress but never completes. In fact the backup has successfully completed but the completion status, and the download link, are never communicated to the user.
Check for this line in logs:
Site export (backup) finished. Filename = temp/filestore/<file id>. File size = 0. Duration = 25243504.
It may be possible to download the file directly from the file store using the file id in the above message. The feasibility of this needs to be confirmed by support.
Note the file is only available for 7 days.