I had the same frustrating problem! How I fixed it was to rename my digital editions folder and move it to another drive then I deleted all instances of My Digital Editions folders on drive C of which there were three. I then ran the installer for version 4.5.6 and it completed and recreated a new digital editions folder and magically populated it with the last 20 or so books that I had borrowed. The program now opens and closes again without error and books can be returned once more.