Cannot open a library book even when reader appears to be authorised

Running Windows 10 (up to date)

Reader Kobo aura (new)

Library in UK Norfolk Digital Library

I have uninstalled Adobe Digital Editions, then installed the latest offering of Adobe Digital Editions.

I have authorised my PC with my Adobe identity.

I have connected my Kobo aura e-reader and authorised it with the same Adobe identity.

I have downloaded a book from Norfolk Digital Library: this book appears in Adobe Digital Editions.

I have copied the same book to my Kobo aura.

When I disconnect the Kobo aura from the PC (properly) and try to open the book on the Kobo aura, I get an error message that the eReader is not authorized to open the book.

Help, please.

Adobe help is beyond useless, sending me in circles without ever getting close to a chat session. This is just a rant, but any advice in dealing with Adobe would also be welcome.