ipv6 access over the internet

[Fate]

> My idea is, that i circumvent all of this and just use the ipv6 to connect to jellyfin.

Good!

> I tested it and it worked over the local network (just like it did with ipv4 before).

Good!

> When testing over the internet it doesnt work.

Not good.

> What is the problem? Is it a ipv6 problem or might it be a firewall problem?

Firewall, both on your router and (if you use it) Windows.
By default, routers and Windows will deny/block/drop all incoming IPv6 connections from Internet.

FWIW: I set up IPv6 on Jellyfin (and already had my router allowing IPv6 through), and I can now access jellyfin over ipv6 from Internet

Agree with everything above.

Just want to add if your IPv6 address begins with FE80 that that is link local address that is not route-able via the internet.

Usually your router allows you to select which IPv6 Addresses are given via DHCP.