During the first setup I have calibrated the frequency readout.
The behavior was as described. Connect A2 to GND - message on screen "Calibrate".
Keep holding A2 to GND rotate the tuning pot for zero beat (using a signal generator).
Release A2 from GND - message on screen "Calibrated"
All working fine after that.
Then I uploaded the sketch that compiles with the new SI5351 libraries.
Calibration has been lost, so I went again through the procedure.
Did not work. Reverted bavk to old, original, sketch and, again, no luck. Cannot calibrate anymore.
When connecting A2 to GND the message is same as if you turn on the power with A2 connected to GND.
You get a tone and you can calibrate for zero beat on that. It is saving something, or at least that is the message, but you do not get the "Calibrated" message anymore and the displayed frequency has the same offset as before.
I can live with this (only 100 Hz), but I would like to understand what happened.