Do you mean that the reload ran and completed normally, but then you could not save it? That sounds like a permissions problem - the service account under which the server runs does not have write access to the document folder.
Or do you mean that it failed just before it would normally complete? The document log might help. Enable the log for the document, and check for the log in the same folder as the document. If the log implies that the reload appeared completed, but it still fails, then issues could be a massive synthetic key, or if writing a file, a file lock or an invalid file location.