Can't copy book from ADE to KOBO H2O 2nd edition

I've paid for a book and can see it in the ADE (v 4.5.5) library but every time I go to copy it onto my Kobo the system seems to crash or at least not respond. Then I see that the authorisation for the kobo is no longer there. I'm new to KOBO and ADE. I have managed to copy over books that were free with no problem. I have windows 10.  All help welcome, thank you