See the question and my original answer on StackOverflow

This is just a rights/permission problem. If you use Excel itself as an end user, it will display a dialog box that say this

You don’t have permission to save in this location. Contact the administrator to obtain permission. Would you like to save in the Documents folder instead?

But, if you run Excel as admin, it will work fine, or it can work, because, at least, it works for me.

Note however that, programmatically, it indeed reports the error with a funny name like CB7A7C80 (it's a temp file it uses behind the scene) instead of the original one (the corruption is only at beginning and end of your string). I do reproduce that funny name also.