If `use_atomic_file_saving` works I'm surprised `use_gio_unsafe_file_saving` does not work, it does the same operations as atomic then tries to restore the permissions, maybe it can't do that for some reason related to the server configuration and throws its toys out of the cot.
I am also not sure why having neither of the `use_` settings set wouldn't work, then saving is just "open/truncate write write write close", maybe somebody could post the error message(s) in `Help->Debug Messages` or on terminal.