The existence of the _MACOSX directory and the ._ files indicate to me that you are using the Mac's internal file compression utility to zip the file and then changing the name to epub. While you can do that on Windows, the Mac requires a 3rd party script to compress epubs properly. Fortunately, one of our members has created a convenient application that both zips and unzips epub files on a Mac and does so without causing any errors.
Here's the link to the thread where he uploaded this application.