An Ubuntu 20.04 laptop installation started not being able to resolve the names of hosts in the same local network. Although the host names were registered with the router, the laptop wouldn’t be able to resolve those host names. Surprisingly, neither netbios resolution worked (although nbtscan would find those hosts).
The solution is to bypass the caching local nameserver. /etc/resolv.conf shows a nameserver at 127.0.0.53. I didn’t find a way to configure that nameserver, since afaik I’m not using bind9 or the likes. What helped was deleting /etc/resolv.conf and replacing it with a link to /run/systemd/resolve/resolv.conf which for my system looks like this:
nameserver 192.168.1.1 nameserver fe80::1%3 search local