Larry,
This is only hypothetical (seasoned with personal experience)

Cannot Get Stream Size (to me) implies it is not getting either the start/end point or the conversion rates to do the math
As tests

Try Save As to another seq filename, then try to convert.
Try a different conversion rate with same codec
Try converting in a different program with the same codec
Try converting to WMA in RB (if it still happens the error is likely in the seq file itself, not the MP3)
Try yelling at it

Make your sound your own!
.. I do not work here, but the benefits are still awesome