This is a very unique issue and I am sure it is due to an internal C9 user account issue…
I have run a C9 account for about 12 months now and am able to clone repositories via SSH from Gitlab.com successfully.
Our company recently setup a new C9 account using the same email address I used om my previous C9 account. This email was first changed on C9. When we now try to create a workspace in C9 by cloning one of the repositories it tells me I need to load the SSH in the repository platform which I have already done.
I have verified that the repository exists and is working properly by logging in on the other C9 account, which syncs to the repository successfully.
I think C9 is getting confused between the two seperate accounts, who seemingly have the same email addresses logged in the background