My home ISP does CGNAT for IPv4, but provides native IPv6. I can use IPv6 just fine to access most of my resources, except for one specific server. I can access the server over IPv4 from my home network, and either over v4 or v6 from other networks I've tried. But I can't access it over IPv6 from my home network.
What could be the problem here? Where do I begin to diagnose it
I kinda think it may be on the isp then, maybe a v6 routing issue? You could work around with an ipv4 tunnel, then route ipv6 through it. If that's possible, I've never done it haha.
Check your ip address config on the destination server. Does it have a public ipv6 address assigned? Does your server routing table have a default route for that ipv6 network? Does your firewall only allow ipv4?