I'm having the same problem with not being able to log in when you start using more features.
I know this extension from chrome and it has a lot of potential once they fix their bugs in firefox.

Hope another update comes soon, Can't stand the dates starting on a sunday!

Everything was working great until I started adding more features (and perhaps even until the latest update). Once I toggled more features on, YNAB has stopped allowing me to login easily. It continues to give me the "Uh-Oh! Something has gone wrong likely having to do with internets and tubes. Refresh your browser to get back in business." screen at login, until I toggle Toolkit off.

I tried to file a bug report, but the bug button does nothing. And then I tried to revert my Toolkit settings back to default, and the red button doesn't do anything either. Would be a fantastic app, and add a ton of useful features to YNAB, but it's still a little buggy. Waiting for another update to make it a little more stable.