No active level possible, but XY calibration OK?!?

[aag 6]

I have now repaired my S5 printhead after a disastrous crash. But I still cannot print anything. The S5 starts active leveling, but stops in mid-air, long before the print bed has reached the nozzles, and gives up with the message that Active Leveling is not possible. I have performed Manual Leveling, twice, but this did not resolve the issue.

Strangely enough, XY calibration seems to work all right, and that includes a round of Active Leveling as well. I conclude that there is nothing intrinsically preventing Active Leveling, and that the sensors triggered by the contact of the nozzle with the glass plate are not broken. What might be the problem then?