I'm running into this too. In my case it's an existing process on a SQL 2000 SP4 box that started erroring sending mail (no error messages that appear useful). I stopped then started the mail session and mail started sending but the message body is always blank even when passing in a normal string. The goal is to email query results in the message body. I'm able to attach them and we're doing that for now as a workaround. I've already tried the following:

http://support.microsoft.com/kb/314304

The registry key that it said to remove doesn't exist and I tried both true and false (and not passing in) @ansi_attachments with no success. It's a production box so I'm working on arranging a reboot but I'm not holding much hope for that. Does anyone have any thoughts?