Ran into error just now and google brought me here. Perhaps you have a co-worker using the same username and same org. That’s probably my issue here as one other has started using C9 and SalesForce charges for licenses ($$$) so we tend to share the same user because licenses can be maxed out so you can’t create your own all the time.
First idea for relief: Log into the wanted dev sandbox or what have you. click Setup. in the left search type Users. Sort Users by “Last Logged In”. You’ll probably find Users that have never even logged into the dev box so deactivate one (not deleting the user, just deactivating) and profit – you have an extra user license. Now make your own User (Admin!) and use that username to create your workspace.
I’d love if someone at C9 would clarify but it might be an edge case so… Anywho hope it helps.