Are you using any markers set in Toast (I assume not, but just in case)? It could cause a problem.
Usually, that error is a problem with the audio or video on the file and the multiplexer doesn't like it. Most likely the file is OK, just something about the content, and the Multiplex needs fixed for it.
You could try deleting everything in the Roxio Convert Items folder, delete the Toast prefs, and try with default settings.