Sometimes to solve this, if given the option, you can switch back to the old (versus the new) uploader. Now that the new MyEBay is mandatory I am not sure if this option is available, but if so try it.
On some computers the new version of the uploader does not work. Try reverting to the Standard Uploader.
On some computers the new version of the uploader does not work. Try reverting to the Standard Uploader.