HI! I have more or less the same problem with my Kobo N647. I can´t register it or sync with the kobo desktop app I writed to kobo customer care and they told me to reset once and once.
After making several resets I´ve lost all the software incluiding in the ereader so I can´t sync or do anything.
I´ve readed that with the image of the firmware on a sd it maybe will Works again so davifor would you mind sending me this image?
PS: This are the last words from Kobo Customer Care:
“Otherwise, I'm sorry to inform you that the eReader reached the end of its useful life. There are forums that you can check that may contain additional information likely to find another way to solve. Rakuten Kobo us, we can not advise or support such solutions or paths.
You can also visit our shop and see the new equipment available to us or you can use your smart phone or your tablet computer as alternative equipment to continue to enjoy reading.”