Hi,
I am pretty sure I found the cause and that moisture is the problem. After dismantling the board I saw moisture inside the ISS housing. I took the board, which also spontaneously indicated the precipitation without the tipping bucket connected, out of the ISS and took it inside the house without battery to let it discharge. (And let it dry) After a few days I reinstalled the battery and connected the tipping bucket to the board and connected it to the console. It works fine and after a tilt of the tipping bucket, the precipitation shows as it should. I tested a lot. Now I have mounted and connected the board outside again in the ISS. Of course as a test I packed the housing in plastic to exclude moisture. It now works more then 2 days as it should and it rains and the precipitation is measured nicely.
So, anyone with this strange problem, first look inside the ISS for moisture. I thougt also that my ISS board was broken, but it is not.
Brgds,
Oebel