Sorry for shortening. “SSH Key” is actually the file path field, exact field name is “Or SSH key file” under " Backup destinations" when “SSH Server” is selected, in the “Destination and format” accordion of an entry in “scheduled backups”.
That field “Or SSH key file” was initially empty, I was using SSH passwords.
After editing and saving the scheduled backup, then running it using the “Backup” action button as a single backup in background, going back to editing the “Scheduled backup”, the password that was under “Login with password” field was now visible in the “Or SSH key file” field, with the “Login with password” field empty.
And even after putting the password back to “Login with password” field and emptying “Or SSH key file” field, and saving, when editing again, the “Login with password” field was again empty and the “Or SSH key file” field contained again the password.
I now switched to SSH certificates, and left password field empty, and it works again, but no way to use SSH passwords anymore on those backups (not a big issue for me as certificates are better anyway, but wanted to report the bug to the Virtualmin team here).