??()
means that gdb can't find symbolic information. This is not necessary for your problem because the stack is already deep in the library code and string/new/malloc are well tested. Start at frame 6 in SymbolCell::get_symbol
to look for problems.
edit: I just noticed that you are actually running lisp-code, so you might have run into some issues with the lisp environment or more likely are running out of memory.