I've just discovered a solution, well it worked for me.
I found it in an old post about an earlier version of Digital editions.
In it the user said that he had gone into his Adobe ID several times and changed various aspects of his account, title, password etc.
It didn't work after the first change, but it did after a few.
So I changed my password twice, allowed Adobe to monitor desk-top usage and added a second email for verification.
It worked and I was able to authorise my computer with the new Adobe ID.