If the problem is returned back, try to use Font_FontMgr::RegisterFont and has the font for the symbol in sources.
static const gp_Pnt MyDefEnd(1,0,0);
static const gp_Pln MyDefPln(gp_Pnt(0,0,0), gp_Dir(0,0,1));
-static const Standard_ExtCharacter MySummSymbol(0x2211);
+static const Standard_ExtCharacter MySummSymbol(0x03A3);
IMPLEMENT_STANDARD_HANDLE(SketcherPrs_LengthDimension, AIS_LengthDimension);
IMPLEMENT_STANDARD_RTTIEXT(SketcherPrs_LengthDimension, AIS_LengthDimension);