My first response to this kind of issue is that the problem is with the font, however the fact that RTF and HTML works and LIT and FB2 fail seems to argue against this. In any case, if there are alternative fonts on the BeBook, try them. If you can add new ones, also try
DejaVu Sans which has a wider range of characters than most free fonts (and is otherwise a drop in replacement for Bitstream Vera fonts).
If it isn't the font, I suggest converting the LIT to PRC (since these display ok, using different reader software on the BeBook than the others). I also suggest using ConvertLIT (or ConvertLIT GUI), rather than ABC converter, and MobiPocket Creator to do the conversion. Explode the LIT to an OEB (HTML, images, and a .opf metafile) and then give the .opf file to MobiPocket Creator. There are several other ways to get from LIT to MOBI, but this is the most flexible.