I recalibrated the KHD pH probe. It took me three tries to calibrate at pH 4: I got a "calibration failed" error message on the first two tries. The calibration at pH 7 worked the first time.

I then took a measurement using the KHD and the Hanna. Hanna 7.7 KH and KHD 6.8 KH...

So I guess it was not a calibration issue. What now?