Are you shutting down all non-essential programs before starting the game, especially anti-virus and any backup or sync programs that may be monitoring the My Documents folder? Check your AV program, if applicable, especially if it may have updated around when this started, and maybe try adding the game's Documents folder and save editor to a whitelist / exceptions list.

Try copying a save to a different folder (like c:\temp\Savegames_patch), and see if that allows the editor to edit and save changes.

Try doing a clean boot and then test the editor. Click Start, or hit WinKey-R, type in msconfig and hit enter; in the General tab, click Selective Startup, uncheck Load startup items (if required) and leave Load system services and Use original boot configuration options checked. Next, click on the Services tab, check the box to Hide all Microsoft services, then click the Disable All button (maybe make a note of which are currently enabled/disabled), then click OK and reboot the computer.
Run msconfig again to switch back to the normal boot configuration.