Forum not reachable over ipv6?

Since a while I’m not able to go to this this forum directly from the bookmark in my browser. I get a message:

Cannot load app
It looks like you are offline! Please check your network connection and try again.

I’m not offline and when I first visit and from there select “discussion forum” under the quick links it works.

When I switch off the IPv6 stack on my PC the bookmark for the forum works too, so I recon there is something amiss with the IPv6 configuration on the server that runs the forum.

No showstopper, but a minor nuisance.

It’s probably this problem in reverse. The server hosting this site doesn’t have an IPv6 address and my Mail-in-a-Box, which hosts the DNS for, is serving its own IPv6 address instead.