I'm not sure why it won't let you save it. Perhaps it's a new feature in 0.92.4. I wonder that because that's not the best place to save a custom template. Maybe the developers finally disallowed it? But here's a better way.
First look in Inkscape Preferences > System > User config. Note that file path, and then browse there with your file manager. If you see a templates folder, that's where you need to save default.svg. If there is a templates folder there, it's probably empty, and that's ok. Just put default.svg in it. If there's not already a templates folder, you can make one.
Doing it this way, you don't overwrite the default, so that if you ever need to get it back again, you can.
If you still have trouble saving it in the proper place, we'll have to ask some further questions. For example, you didn't edit code did you? You just changed the canvas how you want it, right? If you went into the code, it's possible something got messed up?