I get the following message from digital editions 2.0, if i want to activate this computer:
You can only activate this computer with a adobe-id that is not yet been used to activate a computer or mobile device. Please try again.
Almost certainly ADE has got a confused registration, and is now giving a misleading error message about it.
(Basically, it is a rubbish program).
Try deactivating ADE completely, using ctrl-shift-D (cmd-shift-D on Mac).
Restart ADE, and then try again to activate with your Adobe ID.