Every other time this question was asked, the answer was to do BGP.  I'm not familiar with the specifics of what you're trying to do, but I would guess that answer also applies here too.

I have fibre and a 4G connection and a pfsense firewall/router.

With IPv4 I can have failover and use the 4G if the fibre fails (Not likely, but it's mostly for fun^H^H^Hlearning purposes)
The pfsense takes care of this. I then get different public ipv4-addresses, but that doesn't matter, as I'm using NAT anyway.

I have also setup a tunnel on each connection, both to the same HE POP (Germany), but I don't think I can have true failover, because IPv6 is not using NAT, so each internal machine have to have an address in the right subnet.

Unless the other end could route my internal subnet through either connection, whichever was up, and things doesn't work like that out of the box, right?

I guess if I was serious, I'd get a ASN and do BGP, and then the two tunnels should go to different HE POP's.

Or am I missing something?

Can windows and linux get ipv6-addresses in two subnets, and failover if one fails?

Oh, I found the error.

When I changed the endpoint address on HE, I didn't change the interface on my PfSense on which the GIF tunnel ended on.

I fixed it; I made a second tunnel on the same HE endpoint, so I now have two tunnels, one over fibre, one over 4G mobile network.

I have two VPS servers on the outside with IPv6-connectivity I can use to check if needed.

The difference is that the one that works is a 4G mobile connection while the other is over fibre from another company.

If I should have any chance of this getting to work at a company, who doesn't even have any plans for IPv6, to understand, I need something like a traceroute from the HE gateway saying "The GRE packets are being dropped at this ip"

So if one works and the other doesn't,  we have to look at what's different between the two setups

I'd love to get native IPv6 from my iSP. Sadly my ISP is Virgin Media who *still* haven't said when/if they plan to deploy native IPv6 service 

Contact your ISP about native IPv6. You are using a free and shared platform with hundreds if not thousands of other users on a tserv. Speed parity should not be expected. You can also try tuning TCP windowing, MTU, MSS, etc.

How do I debug why the connection is not coming up?

I can ping from outside.

I have just gotten a fibre connection from fibia.dk, with a static IP, and passthrough the fibremodem, but the tunnel doesn't connect.
I also have a 4G from 3.dk, if I use that as endpoint, then the tunnel works, but it is way slower, and metered, so I don't want to use that.

 

I have been using TunnelBroker for a few years now and all the while I have observed that performance (as measured using various tools) over the IPv6 tunnel is very poor compared to my native IPv4 performance; the IPv6 tunnel only delivers around 10% of the throughout compared to IPv4. Since a lot of my stuff uses IPv6 now this is becoming a big issue for me. My ISP is Virgin Media and I have a 200 Mbit downstream / 10 Mbit upstream connection which I monitor rigorously and IPv4 performance is always very good (it does vary a bit based on time of day). IPv6 performance via TunnelBroker is however routinely very poor as noted above. My tunnel is configured on my Apple Airport Extreme (latest generation) router based on the TunnelBroker recommended setup and apart from the performance it works flawlessly. I am connecting to the remote server on 216.66.80.26.

Is this poor performance expected/normal? Is here anything I can do to improve it?

Thanks for any suggestions.