How are you doing your mod? unzip then drag and drop? direct manual file edit? if the code is correct it sounds like a permission problem. for manual edit try using word pad in "Run as Administrator" mode. for extracting files same deal run as administrator if using a different method please elaborate cuz I'm a little slow some days
It does this even when you open WP as admin? also, make sure to preserve file pathway and format when saving it again (i.e. program files/blahblah.bin stays program files/blahblah.bin) If that doesn't work try to save it to desktop as same type then transfer to game data file via drag and drop. If that doesn't work it means I'm outta ideas and it'll probably be either forehead smackingly obvious or stephen hawking class techie gobbeldygook!