2👍
I have opened a ticket for this issue with the cloud9 support @eff M. They are great guys out there and great IDE as well with great support.
Meanwhile try this workaround for the time being:
access your admin page with http://mysite.c9.io/admin/login/?next=/admin/ and not with https://mysite.c9.io/admin/login/?next=/admin/ and see if you can login. it worked well on my side.
Source:stackexchange.com