Myeah... I've had weird issues on my network that I could only resolve by disabling IPv6. Granted, it's probably my fault, but if everything still works fine with ipv4 that's fine to me. One day I will get into it and learn how it work and maybe I'll get it figured out... One day...

Random guess: PMTUD? Like on v4, some people fuck up their PMTUD and are incapable of realizing or fixing it, so you have to have some kind of workaround.

If setting your client machine MTU to 1280 (`ip link set mtu 1280 dev eth0` or equivalent) magically fixes it, that's your problem.