IE itself is not case-sensitive on filenames (though certain editors are). If it were, every file saved by DLTCEP would potentially be invalid (they aren't). Even global names w/in scripts are not case sensitive.
Sometimes the coordinates you click on with ctrl-x are a "few nodes off" from in-game. I went through a bitch of a time getting Jaheira and Minsc to stay in their cells (at Jonny's Dungeon), back in the days of BP-BGT--because what would appear valid when in-game (when I clicked ctrl+x) would not appear in the "exact same place" next game (off by a hair, enough to show up when small prison cells were involved). Mouse-pointer inaccuracy, undoubtedly.
Make doubly sure your coordinates are't any where near a tree, a cliff, a big rock, a lake, and so on (something with a defined search region). This might not be your problem, but I've added hundreds of CRE's the exact same way you have, w/o this issue.