Impossible to copy epub files from ADE on computer to Letto Frontlight.

My Letto Frontlight is connected by USB to my computer and I want to transfer epubs from ADE to Letto Frontlight. But that is impossible. These are the messages I get:

My email adress is ok, the password too, so what is the problem?

I can think of 2 issues. 1) My old computer which crashed had ADE, and 2) might AVAST have something to do with this.
One of the messages says I have to authorize my computer, but it is not possible.

What should I do?