We ourselves had "Error 500" after updating from 12.0.1 to 12.1.0 in our testing servers and we know how frustrating is for everyone to update and see things broken for no reason at all. So we try to document all the issues we find and help whenever we can.
Sorry to read that you went through this issue. It's weird. As far as I'm concerned, I didn't have any server error at all when updating from 12.0.1 to 12.1.0. The update went smoothly. First i updated to the Beta version (12.1.0-B1), later to the Final version (12.1.0) when it was released. (I have both 'Enable auto-update' and 'Force auto-update' set to Off)