Here is a description of a few common error states you may encounter.
Error database not ready
You have not configured the database connection or you have not initialized the database. You can do both by signing into /admin with the super admin credentials.
403 error/Access denied
This can happen if the current user does not have access to a certain page. It can also happen if an admin has turned off a certain feature (e.g.: Contact Us, Password Reset) and a user tries access the corresponding page.
User License Exceeded
This means you are using a version with a user license restriction, and you've somehow exceeded the user count. If this happens, only the super admin can login at /admin to possibly mitigate the situation. You can remove users until you are under the licensed limit, or you can upgrade to an unlimited version.