Suddenly problems with IPv6 ULAs (other IPv6 traffic fine)

[Mr_Maniac]

Hello,

since a few days, I suddenly have problems with my internal IPv6 traffic. I set up dhcp6 via dibbler quite a time ago since I wanted to have local IPv6 addresses with dynamic DNS. My server has a static address in the ULA range (fd00::/ and provides other clients with addresses in the same subnet.

This whole process worked quite fine until a few days ago. The addresses are still being given, but I can't connect between my clients/server via local IPv6 ULA at all... Name resolution works, addresses are given but trying to connect just times out (no matter from/to which machine).

I already checked ip6tables, but the ULA range is being whitelisted. I also tried to check route -6, but unfortunately, I don't exactly know how it SHOULD look like... In route -6, the local address itself is being listed and not a whole network/prefix.

Using the link local addresses works and internet via IPv6 also has no problems. Only ULA...

I may have broken something, when I accidently started dhcpcd (so the DHCP client) on my server while wanting to restart dhcpd. That really mixed up things for a short while (minutes), but after I fixed that, internal IPv6 traffic worked for a few days.

Any ideas what the problem could be or what I could check?
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AMD Ryzen 5900X
64 GB DDR4 RAM
GeForce RTX 3080
Gentoo Linux (most recent stable kernel - amd64)
Windows 11 x64