Adobe Digital Editions (4.0) keeps closing unexpectedly when downloading ebooks from my library

When trying to download an ebook from my library, Adobe Digital Editions (4.0) keeps closing unexpectedly and does not download any content for the ebook (i.e. I can see the title for the book in Adobe, but there is no content). It generates an error report.  I don't know why this keeps happening.  I have an apple computer.  Help please!