I'm having the same problem right now... It used to work, but been a while since used, now get the same notification "error! check activation". Kinda sad this question was asked over a year and half ago, but still not answered. When on Adobe Digital Editions, however, it shows that my computer IS activated....
I am having the same problem as well. I agree with Aurora_North that it is disheartening that this question was asked over a year and a half ago and it is not answered. Whats even worse is I started trying to find the answer to this problem on the help chat and the only answer I got was to check the forums. After finding this thread I posted this link in the chat and the only answer i recieved was that they couldn't help and to check the forums again. The ADE shows that I am activated, but it will not let me open my books. I just hope somebody knows how to help!
I fixed the authorization error with the following registry fix:
1. Close all applications.
2. Choose Start > Run.
3. In the Open text box, type regedit and then press Enter. The Registry Editor opens.
4. In the left pane of the Registry Editor, locate the following registry key: HKEY_CURRENT_USER\Software\Adobe\Adept
5. Select the Adept key.
6. Choose File > Export.
7. In the Export Registry File dialog box, select Selected Branch under Export Range. Enter a name and location for the backup registry key, and then click Save.
8. Right-click the Adept key and choose Delete.
9. In the Confirm Key Delete dialog, click OK.
10. Close the Registry Editor.
11. Open Adobe Digital Editions and reauthorize.