Error message when I try to add a reader "code E_ADEPT_USER_UNKNOW"

Hi,

I first install my account on a Pocketbook. Then I give it to my mother. She has buy a book and creat his own account. Then we want to use only my account so we disconnect his account and try reconnect to mine. It doesn't work so I erase every thing on the pocketbook, and I try to connect again to my account and the message "code E_ADEPT_USER_UNKNOW" appears on my computer when I try to do it.

What mean this error code ?

How can I fix this ?