most likely the problem is related to IPv6 which becomes preferred by more clients over time: Nextcloud at home with IPv4 and IPv6 German and CALDAV calendar: certificate issue in local network
I solved the problem by setting up a local DNS (pihole) and configuring the router to use this DNS server (I had to go the hard way as there is no way to disable Fritzbox internal IPv6 DNS server which always uses DNS server configured in the internet settings)