most of the dozen or so occurences ive seen were spurious interrupts and resets due to fast signals nearby and not so much crystal issues. in the case i just mentioned and most others autorouted fill pretty much served as guard ring for the clock pins. maybe thats why this did not seems to be the cause for us. i do know there were several choices for a fix that worked and narrowed it down to resistor (no cap) right on the rx pin or ground ring. chose the latter because partlist and schematic stayed the same and much easier to refit existing boards.