Recently I get the same issue with my mac (10.11) and evernote 6.4.
This dialog appears every once in a while, although you can simply cancel it and continue.
On my application tab of the my account page, it shows a list of same device (the list can be very long, i just revoked all once):
The only think i can remember is that, i changed my computer name recently. I though this might be the reason, but even if I changed it back, it does not help :/
Please fix the bug, so annoy
Thanks!