Try using the HTTPS/SSL Uploader and see if that doesn't work.  YouTube's standard uploader is not very robust in how it handles transmission errors and when used with certain network topologies file corruption can occur.  Usually this file corruption manifests itself as pixelation artifacts and/or pixel smearing across the screen – but it can also corrupt a file so that it doesn't convert.

    HTTPS/SSL Uploader** ––>  https://upload.youtube.com/my_videos_upload

(** HTTPS/SSL Uploader may look the same as the normal uploader webpage but functions differently.)