I am running emacs25 on Ubuntu 18.04. When I type accents, they display properly, but when I close and then reopen the file they appear like a backslash and then an octal number. This only happens with one particular file: all other files I process with emacs on the same computer and same user account save and display accents fine. How is that possible? Whatever settings are contained in the .emacs config file would apply to all files? I've tried to set the coding system on the individual file that is causing problems, with the command "set-buffer-file-coding-system" and then selecting "utf-8-unix", but I get the message: "utf-8-unix cannot encode these: é"

How can I fix the encoding of this file?

Thanks