Fixed in the next build.
This occurs only when the attachment is an image file that is large enough to require a thumbnail be created. VaultWiki tries to verify that the thumbnail doesn't already exist in the file system, but the function that does this is not visible to the thumbnail DM.
Additionally, most times on your site you were unable to upload anything due to another Javascript that had failed on your site, which I am still debugging. Since the uploader used the same Javascript library as the other script that failed, it was no longer available to the uploader and no upload would occur.