Check your 3G connectivity on your Kindle Paperwhite
Sharing, in case someone else had the same problem. Maybe some people have not even noticed it yet ...
Today I spent a lot of time out of home and brought my Kindle PW with me. In some areas, I found no wifi so was forced to use 3G in order to download a book from cloud. To my surprise, I had no 3G signal. Tried an hour later, different area (or mall, I was actually helping my wife to buy a dress, so boring, lol) no luck. Went to a different area, no signal. Really sad and bored, had no other option that gave up and wait to test at home.
Once at home, I reset the device and tried to register via 3G. To my surprise, even at home, no 3G connection. Long story short, my Kindle PW was never activated at Amazon. A call to Amazon CS uncovered the problem after 10 min on the phone. They activated it via phone.
So, if you own a 3G, check if it actually works. You may not notice it if you register via wifi and haven't tried 3G.