After changing your server role from Scanner to Control Center or to Control Center and Scanner, you may encounter an application error when attempting to log in to the Control Center web interface.
Reviewing the logs reveals the following error:
Access denied for user 'brightmailuser'@'localhost'
A Scanner server does not include a database, while a Control Center server does. When you switch a server from Scanner to Control Center, the newly converted Control Center has no database to connect to. As a result, authentication and application services fail, leading to the error above.
Deploy a new machine configured from the start with one of the following roles:
Control Center
Control Center and Scanner
This ensures the system is built with a proper database and all required components, preventing the access-denied issue.