The blender xfile exporter occassionally puts invalid floating point values into the files. Look out for strings like "-1.#IND00" oder "1.#QNAN0" and handle them as a special case. Your average floating point parser will propably choke on these. But I can't tell if this is the actual reason for the error you encountered. You'll have to use the debugger and see for yourself. Alternatively you might want to take a look at Assimp, a flexible importer library which reads .x files and others. I wrote the xfile loader for this library, and I, too, had to add special parsing for invalid number string for the blender exporter. You might save yourself some work by using this library.