I have a somewhat peculiar problem. I got a new Adobe ID because I wanted to give up the email account associated with my old Adobe ID. Therefore I unregistered the email account and the Adobe ID. Unfortunately I forgot to unauthorize ADE before unregistering its Adobe ID. Now I can't reauthorize ADE using my new ID because I can't unauthorize ADE using my (no more existing) old ID.
So I need some magic from the support team. (Adobe chat sent me here)
I'm answering myself. in case somebody else has got the same problem:
Windows: Delete the folder HKEY_CURRENT_USER\Software\Adobe\Adept\Activation in the Windows Registry.