Foris shows connection test ipv6 error, even though ipv6 works just fine

I’ve created 6in4 ipv6 tunnel, it seems to work fine. The page https://www.test-ipv6.cz on a client shows 10/10. But in Foris “Test Connection” button gives both “IPv6 connectivity: Error” and “IPv6 gateway connectivity: Error”.

What exactly is Foris testing? From Turis command line I can do without problems e.g. ping6 google.com or tracerote6 google.com