@Johann150 So with the help of
@rick I could seemingly pinpoint what the problem is. DNS was always fine and correct. It seems normal that IPv6 NS throws a warn for subdomains. It uses the NS of the domain. (same with your fedi-instance Johann)
Other than that we figured out that the Forwarding from Fritzbox to server was not working correctly
It was the internal "forward" that did not work, so its also not the ISP blocking ports or anything.
The network at home gets disconnected every day by the ISP forceably. Nothing I can do on that. The Fritzbox only allows to postpone this every few hours.
So Each day I get a new v6 prefix (first 64 bits of the adress) but for whatever reason the server also changes its host-id (second 64 bits of the adress) so the whole adress changes every night.
To forward v6 correctly the Fritzbox needs you to enter the interface-id (basically host-id) of the device. The fritzbox can do this automatically for the devices that were on the internet before. (indicated by a green world symbol on the dashboard instead of just green dot).
As for now the host-id of the server changes every night (after being assigned a new prefix...,) and the fritzbox is not able to update the host-id itself. So each night it wants to forward traffic to a non-existing host-id. Resulting in yesterdays errors....
So now I need to find a way to get a static host-id to.that server. DNS will be handled by a script on the server itself. WHAT A PAIN!