Openstack dashboard gives error "Error: Unable to retrieve usage information"

I installed OpenStack on an ec2 instance running Ubuntu 14.04 LTS via devstack. When I login into the dashboard I get an error "Error: Unable to retrieve usage information" When I installed it and logged in for the first time, everything was working fine. But after I stopped my ec2 instance and restarted, I am facing this problem. What might be causing this error? I used the stable juno version of devstack. And the AMI for my ec2 instance is Ubuntu Server 14.04 LTS (HVM), SSD Volume Type