Chris,
This is a known issue that happens once in awhile, and honestly it isn't good news.
What it means, basically, is that due to an -extremely- obscure issue with the activation server, the ID you've used to authorize ADE 1.5 is corrupted (on our end, at the server). We have been trying to figure this out for awhile now, with little success.
You'll need to de-authorize and then re-authorize with a NEW adobeID:
1. Launch Digital Editions 1.5
2. Enter the key-combination of Cmd-Shift-D (Mac OS X) or Ctrl-Shift-D (Windows)
3. At the dialog, confirm that you wish to de-activate the machine
4. Quit Digital Editions
5. Restart ADE, and re-authorize with a NEW AdobeID. It doesn't have to be your Adobe support account ID, it can be anyhting that you can remember.
Now, at this point I'm not sure if your existing ebooks will automatically be re-migrated to the new account. I'm checking with Engineering to find out..
Regards,
Bentley Wolfe
Senior Support Engineer, Flash/Flash Player/Digital Editions
Adobe