Hi Thomas,
I am facing the same problem with Error 500, I managed to solve it. You must first get into your main Odoo page if you have multiple databases and select the correct one (you do not need to log in). Then, you can get into the rating url and you won't see the error anymore.
I know it can be anoying, many of us need to have more than one database running in a single Odoo server but, to solve this, I had to ran a separate Odoo server for each.
I will try to develop a script or a query so you do not need to run several servers. There must be a way to first redirect the user to the database selection and (without him realising) make him select the database and go back to the rating page.
PS: I know you have probably managed to solve this by your own, just wanted to answer in case anyone else is having the same problem. ;D