IPV6 tunnel with route64

[Swtrse]

Hello,

I'm at my wit's end, and Copilot is just sending me around in circles. ^^

I have a PPPoE setup. My "Stone Age" provider (A1) doesn't offer IPv6 on my connection, so I set up a GIF tunnel with route64.
My problem
After rebooting the firewall, IPv6 works for ~120 seconds, then I only get timeouts.

What Copilot suggested and I implemented (without success):
Gateway monitoring disabled
Static route set to the end address of Route64. (Because I'm not using the actual tunnel address, but the also routed /56 subnet.)
An allow-all rule on the interface for the GIF tunnel
Checked the rule on the WAN interface for Protocol 41 ["Reply to" disabled] and set [Status Type] to "No State."
MTU settings and MSS as well.

As soon as my pings only throw timeouts. I don't see anything even in the packet capture.