OpenVPN's IPv6 support is severely lacking…
I've been using OpenVPN for some time now, and basically for a long time, the only way to get IPv6 tunnelled was to run it in layer 2 mode.
The latest versions apparently do IPv6 tunnelling in layer 3 mode, but it's still rather hit-and-miss. Likely, the tunnels are IPv4 only, and if you happen to have a v6 connection either natively or through a migration protocol such as 6-to-4 or Teredo, you're going to find the few (and increasing!) v6-accessible sites will give you away.
Fingers crossed OpenVPN gets something workable going. The only other option is something like OpenConnect (Cisco AnyConnect clone).