I have had the same problem so well described by other reviewers for about a month. It used to be a great add-on! "This addon has effectively been nothing more than a way to log in to the Virtual Cards page of the Capital One site for MANY versions now. It does not create new Virtual Cards and does not allow you to lock or unlock existing Virtual Cards. This is all but useless for anything other than getting you to the Capital One page that allows you to see your existing Virtual Cards. This addon has been the one and only way to create new Virtual Cards since their inception (you can't do that on the website), but now even this addon doesn't do that." When Eno stopped working a year or so ago, Capital One blamed Mozilla and I am sure Mozilla blamed Capital One. I wish they could work together to fix the current issue.
I tried the solution mentioned by Crix, but it didn't work. Not sure what adblock is or how to disable it.