I think your error is different than the known issue described in
"Error: "Failed to load XML.Whitespace is not allowed at this location" in Adobe Digital Editions 1.5 or later (Mac OS X)"
http://www.adobe.com/go/kb406295
Notice that in your case the error message is different. The server code is different too.
Please open a support web case and we'll get somebody to investigate. Also, it might be helpful to know if this happens to just this book, or to all ebooks in your copy of Digital Editions.
To open a Digital Editions 1.5 Technical Support case, please use the contact links at http://www.adobe.com/support/digitaleditions.
Regards,
Bentley Wolfe
Senior Support Engineer, Flash/Flash Player/Digital Editions
Adobe