I don't remember exactly but it has something to do with the text area graphics (appearing behind the level and xp text) in the xp bar. I think I solved it by making those graphics transparent.
Here's the code I used in Black Vellum, where blank.tga is a fully transparent image I made:
Code:
<Mapping ArtAssetID="letterbox_bottom_glass_overlay" FileName="blank.tga" />
<Mapping ArtAssetID="letterbox_bottom_left_cap" FileName="blank.tga" />
<Mapping ArtAssetID="letterbox_bottom_left_mid_cap" FileName="blank.tga" />
<Mapping ArtAssetID="letterbox_bottom_right_cap" FileName="blank.tga" />
<Mapping ArtAssetID="letterbox_bottom_right_mid_cap" FileName="blank.tga" />