Good news, it is working now!
The settings were different for me, in the following way:
- The following are hidden in: “Home Network” → “Network” → Button: “IPv6 Configuration” inside the Section: “IP Addresses”:
Here, in your second picture, Only assign DNS Server was enabled in my FritzBox instead of Assign DNS server, prefix and IPv6 address. I changed and that, plugged out my NexBox from both, Ethernet and power, reconnected, and then it worked.
NOTE: I don’t have the first setting at all. “Internet - Settings” only shows me “AVM-Dienste”, nothing else (see here). I guess this is due to me choosing an IPS with an annoying philosophy… Anyway, this just means that I can’t feedback regarding those settings; they might still be important for others to check.
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At the end of all this; THANKS a lot for your kind and fast support @daringer! I really think that, apart from cool products, you guys also give IT-related average people valuable first technical insights!