You did not mention whether there are any clues in the log.
I would delete all rows from mp_TaskQueue then recycle the app pool, then try again.
I cannot replicate this problem.
Make sure you have specified a valid email from address on the newsletter definition.
I would suggest if you cannot resolve it to try upgrading to the preview build I mentioned previously as a lot of work has been done to improve the newsletter.
Hope it helps,
Joe