This setting came to my mind when I read the help text on the troubleshooting tool under https://fairphone.b2x.com/troubleshooting
Then I remembered that I have turned on the second SIM short before the December update was published. Before this day I used only the physical SIM card. I didn’t think that there would be a link to this problem, but there seams to be a bug.
I will send this info to the support team and hope that they can help me and that they will respond to my messages.
I will keep you updated, thanks for your help.