First you can check Access Group instances and confirm if it is deleted or not.
As a workaround you can try login using firstname.lastname@example.org and add a new Access Group with same name or a different name like "Revenue:AdministratorNew". In the Access group select your application and version. Select default portal as Developer. Add required Admin roles. This way you should be able to continue your work by adding operators to this new Access Group.
Please see some discussions also on similar issue,