Does running `ip link set mtu 1280 dev eth0` on the client machine fix it?

A lot of servers have somehow managed to screw up their path MTU discovery. People have been using client-side workarounds for this for many years, but I suspect the workarounds are often forgotten on v6. Unfortunately the resulting problems then get blamed on v6.

The other possibility is broken multicast on your local network. Some Wifi mesh APs and "range extenders" just don't work properly. The test for this would probably be to take them out of the network path by connecting directly to the main router via Ethernet and seeing if you can talk to Internet hosts properly.