After the upgrade I had to reset the router to factory settings for the error to disappear as you mentioned. Now it works! Thanks!
One possible cause of the problem might have been that I first configured the router without a mobile connection (no SIM). After the router had been on for a while I received a SIM-card and configured that afterwards. I'm guessing that can be a possible cause since the setup wizard behaved differently with the SIM-card inserted.