Looks like known_hosts will store hostname in lowercase, so do not enter them in upper or mixed case in the gui.
As I had to re-install freedombox due to my own carelessness (don’t ask it was stupid) I was thinking remote backup sounded like a very good idea.
On setting up I entered the remote location like this user@HOST3AB4.com.
this lead to the following error “HOST3AB4.com not found in known_hosts …” on checking it, I found that it was there (in /var/lib/plinth/.ssh/known_hosts) but it was there in lower case. on a whim i tried entering the hoste name all in lower case (host3ab4.com) and lo and behold works like a charm
Maybe automatically convert the entry from the user to all-lowercase?