Sometimes ADE gets its registration/activation confused and in a semi-authorized state. Uninstalling and reinstalling does not help. Unfortunately, it often then gives misleading error messages about what is wrong. A common incorrect message informs you that the ID is already in use on another computer and cannot be reused. *** This can often be resolved by completely removing any authorization using ctrl-shift-D to the Library screen on ADE (cmd-shift-D if on Mac). Restart ADE, and then reauthorize with your (old) Adobe ID. In extreme cases on the mac, the following extra step has helped some people. Navigate to /Users//Library/Application Support/Adobe/Digital Editions and drag the activation.dat file to the trash. If you are using 10.7, see Access hidden user library files | Mac OS 10.7 Lion. http://forums.adobe.com/thread/1265248?tstart=0
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*** Adobe have only known about this bug for just over two years, so don't expect a fix for at least two more.