FritzBox 7362 SL accidentially accessible via IPv6

Question

I have a FritzBox 7362 SL running with the following config:

• DynDNS (only IPv4 address mapping)
• port exposure to a local web server (ports 80 and 443)
• remote access via MyFritz activated
• no other remote access or VPN activated

Everything worked fine for years. Today I changed registered a new DynDNS and adjusted the settings for the FritzBox. The new DynDNS now also supports mapping to IPv6 addresses.

After that, my webserver isn't accessible anymore. Instead, now the login page of the FritzBox appeares, for both, HTTP and HTTPS. Login is refused, thank godness, even with valid credentials.

I found out, that this is only true when the IPv6 address or the new DNS name is applied. The DNS name now also maps to the IPv6 address. Applying the IPv4 address accesses my webserver (when reactivating the port exposure)

I triple checked all the FritzBox' setting; there is definitely no active remote access setting left. I also removed the port forwarding setting and the MyFritz exposure (and restarted the box, of course).

Does anyone have an idea about this?