As a temporary fix, instead of service systemd-resolved restart as per the article, you can try this, service systemd-resolved stop
Once the service is stopped the port should be free. You’ll have to do this on every reboot (though maybe you can try adding the command to /etc/rc.local to stop it on every reboot)
Yep, it might be enough to just add that file with the setting set to no and restart.
Because the Fritzbox uses a DS-Lite tunnel.
Thanks, that pointed me in the right direction!
If I’m understanding https://en.avm.de/service/knowledge-base/dok/FRITZ-Box-3490/1611_What-is-DS-Lite-and-how-does-it-work/ and https://superuser.com/questions/1301857/using-pcp-port-control-protocol-in-practice correctly it seems that it’s technically via PCP (Port Control Protocol) that this is known, rather than DS Lite per se, but also that PCP only comes into play here because DS Lite is being used.
(Why point out the distinction? For future readers. I can imagine some braindead ISP somewhere (likely a super cheap reseller) offering DS Lite but then not knowing about PCP, and either not offering port forwarding at all - or they do but you have to fill out a form and snail mail them and then they snail mail you back a printed letter containing a list of port mappings.)
So, here’s a page from the online manual that specifies how to do this specifically for the FritzBox 7530
Based on the original post though I am 100% sure that OP has already seen this page, already tried it, and therefore knows that the warning under 2.10.b. applies to the OP’s case (i.e. FritzBox doesn’t allow it from UI because the ISP doesn’t allow it - that honestly had me wondering just how the FritzBox knows the ISP doesn’t allow it, but that’s a different topic).
And one can prototype this for free by using something like localhost.run or ngrok.com
The explanation is pretty boring. If you look at https://superuser.com/questions/421997/what-is-a-ssh-key-fingerprint-and-how-is-it-generated it’s explained that some fingerprints are displayed with Base64, which according to https://en.wikipedia.org/wiki/Base64 allows the use of all 26 letters of the alphabet, and both the complete uppercase and lowercase sets.
So basically it’s just random chance that a given fingerprint has some data that shows up as a word.
SSH keys can likewise use base64, e.g. for PEM format, as per https://unix.stackexchange.com/questions/492704/what-encoding-is-used-for-the-keys-when-using-ssh-keygen-t-rsa