If you're getting that, "Unkown Error" message on your reader... try downloading the firmware update from the company's website.
I was having problems with my e reader. it was crashing halfway through one of my books. another thing, if I have books that I purchased from other websites ( google, CAlibre) and I sync with the kobo app, only the kobo bought books remain and the others disappear.
I have to use adobe dgitial editions to put books on my reader.