Packet loss when connecting via IPv6

[Johannes Brechtmann]

When connecting to a vpn server via IPv6, I get only small packets (I
guess), so ping works fine, http redirects work but http 200 responses
get lost. With ssh I can log into a server and use basic commands like
uptime but e.g. top doesn't work.

If I connect to the server via IPv4 everything works fine.
If I connect to the same server with the Cisco AnyConnect client (no
matter if IPv6 or IPv4), everything works fine, too. 

I'm using openconnect v7.06 on Debian testing and openconnect for
android v1.11.

I'm wondering if this is a problem of openconnect or the server I try
to connect to.

Can anyone confirm or deny this behaviour with any vpn server?

regards,
Johannes