This seems to be a slight misfeature in Mark IV: I take that when opening a font file with an .otf extension, it assumes it is an OpenType font with PostScript outlines (cubic splines); when the file has a .ttf extension, it supposes there are TrueType outlines (quadratic splines). Both types of outlines are mutually exclusive and are contained in different tables of the font file (respectively, 'CFF' and 'glyf'). This is indeed a widespread convention (including FontForge's), but this font doesn't follow it, since it has TrueType outlines and an .otf extension; hence the error message (ConTeXt is looking for a CFF table and can't find it, since there is a glyf table instead). When you edited the font in Fontforge and exported it as UnicodeSymbols.otf, it was converted to a CFF-based OpenType font, so ConTeXt could use it. I consider this a bug in Mark IV (I think the problem is really in ConTeXt, not LuaTeX). It should be able to use the original font, even if this extension thing is annoying.
And How can I quicky access symbols like U+25c9 ?
One would think you knew that, Luigi! Just use ^^^^25c9, for example. Arthur