Your own tests have ruled out a server issue so that at least removes that from the equation.
It's obviously something to do with this install, we just need to find out what.
You've uploaded the files again so that should mean all of the files are correct.
It can't really be a JavaScript issue as the uploader uses Flash.
Have you made any other template changes apart from for the add-ons?
Are there any outdated templates, although again that shouldn't really affect this.