I installed this add-on and like so many others I got VERY frustrated with the "unknown error" in the sign-in box.
So I did some googling, and found the suggestion regarding AdBlock below (adding the @@||http://evernote.com^ exception); however, this did not help.
So I went one step further, and completely disabled AdBlock.

The sign-in worked flawlessly.

After that, I figured I'd re-enable AdBlock, only to find that suddenly WebClipper thought I had no Internet access. So I pulled up the filter preferences, and searched the whole set of list subscriptions for 'evernote.'
I believe there were four different entries, not including the exception rule I'd made.
Disabling those 4 list entries solved all the issues I was having.

I hope this helps other users that are frustrated with this add-on and that the dev's take a moment to let these frustrated users know that the add-on is 100% usable, just not automatic when paired with several (extremely common) adblocking add-ons and lists that are out there.

Hopefully there's a way to adjust the add-on's permissions or path such that it doesn't get stuck in this dead-end. At least a "no connection available. Are you connected to the internet? are you using an adblocker?" dialog that directs users to an Evernote how-to for disabling adblocking for Evernote functions.
Thanks for making this app!