Serial cable won't physically connect?
Just unscrew the protruding nuts on one side.  You'd think there would be a standard, but there only sortof is and it's only sortof followed.  This happens occasionally.

Most of the time, unscrewing the nuts, which are usually jack-screws, won't do any harm.  I have seen cases where the nuts held the front plate on or even held things together inside.  In those cases, try the other connector instead.  Even if a connector comes undone, it won't hurt the signals electrically.  Worst case, it falls apart and you wrap some electrical tape around the exposed conducting parts.