Troubleshooting Kindle 4 - Factory reset does not work

Hey there,

so I have a really weird problem. My mom bought an Kindle 4 (used) on ebay.
The problem is now, that the kindle stays connected to the old kindle user.
And whats really weird is that nothing helps against it.
My mom was already in contact with the amazon support and they helped her alot, but nothing helped in the end.

Factory Reset -> Reboot and then automatically reconnects with the old user
Enable flight mode, factory reset -> Old user
Deleting the content (books etc.) over windows -> Reconnect and redownload

Even with flight mode enabled the books and the old user just keep comming back. I dont know how the kindle does this with no connection at all.
There has to be a hidden directory or something like that. But where?

I cant logout the user over the kindle, it just says there is a problem and i have to try again later.

So... what can I do now?

Firmware Version: 4.1.3