When do they break? Presumably on tow, but at what point?

At 22:11 09 July 2008, Jim Kellett wrote:
We've been using a Tost reel internal to the towplane (PA-25) for
several years now, and are delighted with the overall improvement in
operational efficiency and safety. However, we've also had over a
dozen breaks in the Tost-supplied rope/cable, always right where it
enters the "slug" that contains the ring that connects to the glider.
It breaks - the knot inside the "slug" is intact; we have never broken
a weak link (sized properly for the gliders we fly), and several
breaks have occurred with relatively low-time (e.g., fewer than 300
launches) cables. Any clue as to what's going on here and how to
prevent it?