I had the same Problem..
I installed RabbitMQ and Enabled Web Interface also but still couldn’t sign in with any user i newly created, this is because you need to be administrator to access this.
Do not create any config file and mess with it..
This is what i did then,
Add a new/fresh user, say user
rabbitmqctl add_user test test
Give administrative access to the new user:
rabbitmqctl set_user_tags test administrator
Set permission to newly created user:
rabbitmqctl set_permissions -p / test ".*" ".*" ".*"
That’s it, enjoy 🙂