Hi! I accidently posted my question in the Adobe Reader forum (http://forums.adobe.com/thread/1181216?tstart=0 if you want to have a look), and they told me that the best way to get this to work would be by re-authenticate my eReader.
So my problem is that I authenticated my eReader (Sony PRS-650) with some random Adobe account for a long time ago, and I have no idea about which it was (and yes, I have tried several ones), and it feels like the best way would be to re-authenticate my reader, because I want to be able to use it with epub files.