Note that the "Operator" role can be edited and the permission "Studio Data Management - Develop" now called "Studio - Studio Develop" may not remain linked to it. The customer may for instance have a custom role more congruent than linking "operator" with developer.
So make sure the user has the permission, not just the role.
Also check the user's "Profile Preferences" and see that the "Studio user name" exists to confirm the permission is assigned; this is the username for logging in to Studio and may differ from their email used to login to Cloud.