While looking for the solution to another problem, I came across this gem:
If you have problems posting a message or replying to a message board (the message isn't posted and you are redirected to another Web page), it may be because your Internet Service Provider has a very short timeout setting that does not allow our servers to complete the posting process. This setting is controlled by your ISP. It is not something you or MSN Groups can adjust. However, we do suggest that you clear out your cache on a regular basis.