from flask import render_template
@app.errorhandler(404)
def page_not_found(e): # e must be in there # note that we set the 404 status, this is what it catches return render_template('404.html'), 404

I had the same issue. I had it because I changed the parameters SERVER_NAMEof the config to a name which is not the name of the server.
You can solve this bug by removing SERVER_NAME from the config if you have it.