Activation servor error - E_ADEPT_INTERNAL

Had version 4.0 on my Mac, which worked some of the time, but would crash repeatedly with certain public library books. On the advice of another forum, switched to v 3.0; now I can see all my books, but when I try to move or open files (either newly downloaded or existing), I get the above-mentioned error.

The books that were successfully downloaded are still available on my nook, but I can't open them with ADE.

...help?