As the new feature subject was closed I better open a problem for this.
Database revision: 446.
After I installed this new release, I got the 10-stars as default.
So I switched back to 5-stars again.
However now the ratings have halved.
I get only (±) half the number of stars but also the average-ratings are halved.
Just to be sure I did a "Recalculate ratings." but without the required result.