How to correctly configure IPv6 with OpenVPN?

I think you need to proxy NDP requests to your public IPv6 addresses. I haven't tested this personally, but this is the theory:

Your ISP will send traffic for your whole IPv6 network (2a04:52c0:101:xxx::/64) to your server. This means that, when someone on the Internet tries to connect to an IP address inside that network, the traffic will be sent to your server, expecting it knows how to handle it.

Your server has an address in that network (2a04:52c0:101:xxx::100). When it receives traffic to another address, it ignores it, because it's not an address that it can identify with. So, the traffic that goes to the devices that get an IP address from that network assigned by OpenVPN stops dead on your server.

To make your server realize that it has to get that traffic and sent it "down" through OpenVPN, you have two options: use a different IPv6 network for your OpenVPN clients (so traditional routing works) or proxy the traffic to your current network. The first is the best option, but your ISP may not assign you more than one /64; the latter is NDP proxying, akin to ARP proxying in IPv4.

Using NDP proxying, your server will get the traffic for addresses not its own and resend it to the clients with that same IP address connected through OpenVPN. You will have to do this for every IP address in your network that belongs to an OpenVPN client.

There are other answers in the StackExchange network that cover this in detail:

• IPv6 routing problem
• How to asign full IPv6 subnet to OpenVPN client

Please, check those answers for a more thorough explanation.