Have been unable to find any answer to this question! Obviously seeing the amount of times this question has been asked it should be stuck somewhere that it is easy to find!! But No!
Still not had a reply, even if you moved it to the correct forum! I spent 40 minutes with adobe customer help desk who were totally rubbish and after wasting my time said " can't help you, you need to come to the forums page"!
Fat lot of good this has been!
Sorry for the inconvenience caused.
Please refer the troubleshooting steps provided in this help document: Error messages and troubleshooting tips for activation errors in Adobe Digital Editions
Check if it helps.