Cannot create Cloud9 environment - networking or ssh key error?

I have an existing EC2 instance and I'm trying to create a cloud9 environment for connecting to it. I have copied the ssh key info into the ~/.ssh/authorized_keys file, and I'm using the default user ubuntu. The security group associated with the instance is set up to allow port 22 traffic from the internal AWS subnet for my region. Every time I try to create the environment I get the following 2 errors:

"Your request failed because there’s a conflict with current state of the target resource. Please change the properties and try again." "AWS Cloud9 couldn't connect to SSH server ubuntu@<ip_address>"