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.