I just had the same problem and finally found a solution. Connet your nook to the computer - not through digital editions. Open the nook 'drive' and delete the digital editions and adobe digital editions library. Then eject the nook. Reconnect the nook and open digital editions 2.0. The PC and nook were reauthorized and I was able to transfer a library bood to the nook with no problem.
We were having a similar problem (same message) with a Pandigital Novel. After reading the previous answers, I managed to fix the problem this way:
I turned on the e-reader, connected the USB cable to my PC, and mounted the reader's "drive" without using the Adobe software. (My PC is a Ubuntu Linux machine.)
I think there were two expired library ebooks "stuck" in the reader.
I erased these files:
Digital Editions/Annotations/Digital Editions/Sew_Serendipity_Bags.epub.annot Digital Editions/Annotations/Digital Editions/Ties_That_Bind.epub.annot
Digital Editions/Thumbnails/Digital Editions/Sew_Serendipity_Bags.epub.png
I used a text editor to examine and modify this file:
The manifest file has book entries bracketed by <de:contentRecord> and </de:contentRecord>. I saved a copy of the file and then removed the two expired books. I also cleaned out duplicate entries. (The free books were listed twice.)
After saving the file back to the ereader, I disconnected the USB cable and checked the bookshelves. The Adobe bookshelf was now empty. The Kobo bookshelf had the free books plus the two I had tried to remove. A restart might have cleared this, but I used the Kobo bookshelf management to make them go away there, too. I then shut the reader down.
After bringing the reader back up and attaching it to the Windows PC Adobe Editions, we were again able to get books from the library and put them on the reader.
It should be noted (for Google) that the error message is
Program: ...File:...\..\src\de17_lib_library.cpp Line 215
I upgraded my laptop from an old Sony Vaio running XP 32-bit to an HP running Windows 8 64-bit and received the same assertion failed message. The upgrade also included going from ADE 1.7 to ADE 2.0. Hitting "Retry" cleared the message from the screen, but everything ran slow, only 199 of the 230+ books on the nook showed up, and I was unable to load new books to either the nook or ADE. De-authorizing and re-authorizing both devices changed nothing.
Here is what did work. I first connected the nook to the pc without starting ADE. I then opened Explorer > Computer > MyNOOKcolor (yours may be different). I backed up (copied) the files ".adobe-digital-editions" and "Digital Editions" to a folder on my desktop and then deleted them from the nook. I then "Ejected" and "Safely Removed" the nook from the pc. I am not sure if removing the nook was necessary but I wanted all "communication" between the devices to be "fresh". I re-attached the nook and made sure that the pc recognized it. Then I started ADE and "Hooray!", no more assertion failed message. It took a few minutes for ADE to show all the books loaded on the nook but it did show all of them this time. And I can now download and transfer books. Good luck and good reading.