There appeared a notice at my startpage. One of my members should have added some changes to the facts of another. Looking there, I saw the email adress of the first member writte as living town of the second one. And at the right list at the start page the first member was listed with the birthday of the second one.
I asked him what he had changed. The answer was that he only sent a birthday greeting to the second member.
After uploading some changes, the effects written above were gone.