Can't resolve domain names after upgrading to Debian 12

After upgrading Debian11/KDE to Debian12, restarting and running sudo apt-get upgrade it shows errors like Could not resolve ftp.XX.debian.org. These also show when running sudo apt-get update. I then tried to open websites in the Firefox-esr browser and it can't open any (it shows the "Hmm. We're having trouble finding that site." error). I can't ping any sites either, it shows "*Name or service not known*". So it has problems resolving domain names with DNS. ---- **Details** and what I tried: I tried sudo mv /etc/resolv.conf /etc/backup.resolv.conf. DNS still works on a Debian11 machine and it worked before upgrading to Debian12. The nftables firewall rules are the same as before. The time was off by minutes again but I corrected it so it shouldn't be off by more than seconds. At the end of upgrading at 99% I tried to open the browser when it asked me to replace a certain config file, this caused a black screen (once during updating the screen could not get woken up too) and logged me out so I had to finish upgrading with sudo dpkg --configure -a which seemed to have worked. Maybe I need to check if the upgrading worked. Right now I can't use the Internet on that machine while NetworkManager displays it's properly connected and my router page also shows the device as connected. * grep ^hosts /etc/nsswitch.conf shows hosts: files mdns4_minimal [NOTFOUND=return] dns mymachines * /etc/resolv.conf contains #Generated by NetworkManager nameserver: 1.1.1.1 (I already tried adding nameserver 1.0.0.1 beneath it which didn't help) * nmcli c show | grep -i dns shows the below for the Internet connection (not the VPN connection). On the Debian11 machine where DNS still works those values are different: it does not have connection.dns-over-tls. I think **dns-over-tls** likely has to do with the problem. It's also configured in the router that is used by multiple machines of which only the Debian12 machine can't reach websites. I use IPv4-only for good reasons and a VPN.

connection.mdns:                        -1 (default)
connection.dns-over-tls:                -1 (default)
ipv4.dns:                               1.1.1.1
ipv4.dns-search:                        --
ipv4.dns-options:                       --
ipv4.dns-priority:                      0
ipv4.ignore-auto-dns:                   yes
ipv6.dns:                               --
ipv6.dns-search:                        --
ipv6.dns-options:                       --
ipv6.dns-priority:                      0
ipv6.ignore-auto-dns:                   no
IP4.DNS:                             1.1.1.1

----- **Why** is that and how to solve this problem?