CODE E_AUTH_BAD_DEVICE_KEY

I have been using ADE with my KOBO ereader since August. Suddenly, about three weeks ago, it stopped being able to download books from my library. And I get this message.

I have searched and found it is a common problem. Solutions suggested include:

Use version three - same problem

Go into the keychain access and change the access - same problem

Drag out the .dat file - same problem.

Please, is there a definitive resolution? At the moment, thanks to ADE, I have a very expensive paperweight.