Device authorization problem - help!

I've tried authorizing my kobo (after resetting to factory settings because of a problem in transferring  library books). Now I get "this device cannot be authorized because you have reached the limit of devices...". This is not a new device!!!

I've had repeated problems over the last few years in transferring books from the library; I've usually  resolved it by resetting my kobo to factory settings, remove authorization from ADE, then reauthorizing ADE and the Kobo; but, this time, it won't even let me authorize my Kobo .....