I received a SQL content database backup file (BAK) from the customer. I restored it in SQL Management Studio. I set the farm account DB owner for this content database.
I try to set in CA the site collection administrator. But it doesn't work. The input field is empty/greyed out to set some username.
In the ULS I see this error:
Access denied. You do not have permission to perform this action or access this resource.
Who knows what the issue is? I am using SharePoint 2013 on premise.