The on-off button became harder and harder to press. So I looked at it with a powerful hand-lens and immediately saw the problem. Lime scale buildup, with visible crystals forming in the narrow space between the body and the button. A descaling chemical solved the problem immediately! Some fizzing, and a lot of white solution being flushed out as I pressed the button repeatedly while irrigating the button with the descaling chemical.

Operation completely back to normal.