Before the upgrade, the public directory contained an empty file index.html and subdirectories site and journals/1 that contained images and our style file.
After the upgrade everything except index.html was gone.
Did you perhaps start the upgrade with a 3.1.2.4 source download and forget to copy the public/ directory over from your old 3.1.1.4 installation? You’d need to migrate that over by hand, since it won’t happen automatically.