Coincidentally, I also ran into this after adding login capabilities on my flask app, but I didn't investigate further, and I wouldn't think they would be related. It worked after I changed the button to a form with a submit button, where the form action is url_for('contact_add'), like this:
<form method="get" action="{{url_for('contact_add')}}">
<input type="submit" value="Add a New Contact">
</form>
I didn't actually try running this, so I might have a syntax error or something.