The upload of a file (as well as the upload of a page) fails often due to loss of session data.
In such cases, the file upload form remembers all text description fields including the new file name, but forgets the previously selected file path with original file name. It's a bit bothering to be forced to select the file again and again.
I think this might be impossible to fix due to limitations of HTML. It's impossible to set the value of a file input (it can only be read), because that could potentially let the website "steal" arbitrary files from the user's computer.
Maybe try using UploadWizard rather than the Upload form? I think it doesn't have this issue, as it can check for the loss of session data from JavaScript and re-submit the files without refreshing the whole page.