Thank you, sdmitch.

My Sketchup(2014) is configured to use decimal comma instead of point (the European way). If I make L-stairs/Open, everything works fine repeatedly. But if I make L-stairs/in-situ, some numbers are written to Stair Maker.txt with decimal points and Sketchup doesn't like that, so Stair Maker stops working. If I change those decimal points to decimal commas in Stair Maker.txt and save it, Stair Maker works again. I think this is the issue here: some internally calculated results are written to Stair Maker.txt with points instead of commas.