After the jump at the end of the concert you can see him leaving the stage with his hands on his back, as if it was hurting. Maybe it's just some general bone disease, afterall they're getting old. I wouldn't say it's serious tho, he seems fine in the latest short performances so maybe he just gets tired after a long time standing.
In general I could also see how lately not just him but the whole band kind of seems to be lacking in energy and motivation.