I have the same issue here on an old install. Updated to file editor 5.7.0 and suddenly I have an ‘access denied’ message when trying to access configuration.yaml
I use HAOS, so maybe it is the HA Core that is the cause.
I think there were a change to the config library in one of the recent updates and I can see that mine now is called homeassistant instead and the config library is just a symlink to this.
If the permissions are set wrongly on the symlink, then that might be the cause.
FIXED
Thanks Wally, I uninstalled file editor, reinstalled and it gave me the same message for configuration.yaml. Then I clicked on the folder (top-left) icon and chose to open a non-sensitive file: HA_VERSION, and that is what did it for me. Fixed permissions with other files x