The page contained a custom script to enforce the promotion choice, for which the rules were outside the Diagram's standard options at the time this Diagram was created.Newer versions of the Diagram script could have supported it through morphing and multiple promotion sets, but it appeared a recent change in that area made it mistake some promotion codes (as well as capture-the-flag goal squares) for forbidding access to the square. And this was then triggered by that this custom promotion routine checked whether the choice was appropriate for the destination rank, but it did not check whether the piece actually was a Pawn. So it would try to promote all pieces on the last three ranks once I fixed the other error.
This should be fixed now.
The page contained a custom script to enforce the promotion choice, for which the rules were outside the Diagram's standard options at the time this Diagram was created.Newer versions of the Diagram script could have supported it through morphing and multiple promotion sets, but it appeared a recent change in that area made it mistake some promotion codes (as well as capture-the-flag goal squares) for forbidding access to the square. And this was then triggered by that this custom promotion routine checked whether the choice was appropriate for the destination rank, but it did not check whether the piece actually was a Pawn. So it would try to promote all pieces on the last three ranks once I fixed the other error.