I'd probably convert the document to a RTF or simple text file. This should reveal any potentially illegal characters, which are probably the problem. Word documents can contain a lot of garbage as well as some typographic trickery which "proper" tools such as Encore or other Adobe apps cannot handle.
Good suggestion. We did the "cleansing" of the Word docs and then discoverd that small blank spaces are required between subtitles periodically. No rhyme or reason but the workaround worked when errors were generated while burning to DVD.