Debugging upload problems on JoomGallery

After installing JoomGallery and writing a migration script to migrate from Menalto Gallery2 (G2) , I finally got to uploading photos. But for some mysterious reason, every attempt to upload images would fail.

The manual upload did not work (blank page), the FTP upload did not work (blank page), the batch upload did not work (blank page or session timeout sending me back to the login) and the Java upload did not work either: 

wjhk.jupload2.exception.JUploadExceptionUploadFailed: wjhk.jupload2.policies.PictureUploadPolicy.checkUploadSuccess(): The string "JOOMGALLERYUPLOADSUCCESS" was not found in the response body