I've tried a few different mesh fixing tools over the past couple of weeks (Netfabb, Fusion360, Meshmixer, Microsoft), but all of them do this same thing. No matter which program I use, it's always the same: initially it looks fine in the preview, but as soon as you try to slice it you get this hole in the waist.
(Picture 1 is a sliced version of the unfixed model; Picture 2 is after fixing.)