Thanks to Stuart Eustice for his assistance after my crash. Resting a while after the crash was not a good option because the rig was drifting away from the shore quite fast.

The back footstrap screw on my SP40 board disconected from the board in the beginning of the run. Basically the plastic thread failed.

Initially I thought that I missed the footstrap with my back foot and continued sailing. Then I looked at my back footstrap and discovered that it was disconnected at one end, not holding my foot at all. By that time I reached speed about 36+ knots and the chop and then crashed, landing some 20m away from the board.

My experimental "safety harness line" promptly disconnected me from the rig and there was no injury (apart from slight dizziness for a few minutes) and no equipment damage.