Quite likely a case of ADE getting things confused, and then giving a misleading error message.
Try clearing up any idea of activation your copy of ADE has. Keep the ereader disconnected for this.
Then start ADE and hit ctrl-shift-D (cmd-shift-D if a Mac) to forget authorization on ADE. Now restart ADE and reregister with your Adobe ID.
IF that works, then do similar with your device connected.
Then start ADE and hit ctrl-shift-E (cmd-shift-E if a Mac) to forget authorization on the device. Now restart ADE and reregister the device.
With this method, all works now, thanks for the help.
ADE must be very near the top of my 'awful program of the decade' awards; and the surrounding infrastructure is slightly worse.
It's a shame that it is a necessary part of library eBook reading or buying eBooks for many people; with Kindle lockin almost the only alternative (not an alternative for European library book users).