If a base <Role> Default Group is no longer available, likely caused by manually deleting the role group directly from the CM Database, the error will occur.
A possible scenario is that a new Role Group is created and renamed to match the name of the base Role Group. The base Role Group is then renamed to something different. After that, all the Roles are moved to the new Role Group. This creates a situation where the empty base Role Group cannot be deleted despite having a name that looks like it is not the base Role Group. Since a base Role Group cannot be deleted, an incorrect workaround may be used to delete the Role Group directly from the CM Database. As a result, the base Role Group is now missing and causes the service to fail on startup.