When this occurs, ensure the REUser account is not locked on the SQL Server.
  1. Login to the server and open the SQL Server Management Studio
  2. Connect to the SQL Instance that the database with the issue is attached to
  3. Go to Security > Logins > REUser and open this
  4. Under 'Select a Page' click on Status
  5. Under 'Status SQL Server Authentication', ensure the box "Login is Locked Out" is not checked
Once this is unlocked, the error should be resolved.