I’m not familiar with LXC but it sounds similar to IPv6 only, can't connect to itself - please review your container config and verify it has valid outbound IPv6 connection (public IP, routes, firewall etc.)
I found additional steps might be required to enable IPv6 in Proxmox and LXC
- No IPV6 for LXC container | Proxmox Support Forum
- IPv6 without NAT inside LXC container - LXC - Linux Containers Forum
to verify IPv6 works inside of the container run curl -v --ipv6 https://apps.nextcloud.com which should produce similar output
* Trying [2a01:4f9:6a:1de8::2]:443...
* Connected to apps.nextcloud.com (2a01:4f9:6a:1de8::2) port 443 (#0)
* ALPN: offers h2,http/1.1
* TLSv1.3 (OUT), TLS handshake, Client hello (1):
* CAfile: /etc/ssl/certs/ca-certificates.crt
...