The tenor Jonas Kaufmann has again withdrawn from the role of Siegmund in the Metropolitan Opera’s production of “Die Walküre” by Richard Wagner. Stuart Skelton will take over in Monday’s performance, the Met said on Wednesday, because Mr. Kaufmann is ill. Frank van Aken replaced Mr. Kaufmann on Saturday, placing him on stage opposite his real-life wife, Eva-Maria Westbroek, who is singing Sieglinde.