As many other users mentioned, this stopped working the last time Capital One put out a new version about a month ago.
For me, when I click on the Eno logo and "Checking Session" appears, if I need to sign in a new window pops up for the login but then immediately shrinks and closes with out giving me the opportunity to put in my credentials. I was able to make it work again for 2 uses by uninstalling and reinstalling the add-on. Also, the work arounds others mention, go sign into C.Ones website, then open a new tab (or three) on the same retailer allow me to struggle through.
Honestly, I may drop Capital One if this isn't fixed, there are competitors with apps that work.