All I can tell you is that my rudder is designed with two layers of double bias carbon wrapped around it over the leading edge with an extra one on the head. But this is a daggerboard rudders and doesn't have the cut out your swing rudder has.
If your rudder is built along the same lines where the core is cut out of the leading edge in the head (to make sure it kicks forward enough when down) they would most likely have to cut the Double Bias carbon and wrap each end of the cut fabric over itself.
The stress fractures that you see in the gelcoat are the same pattern as the DB carbon and may indicate the core is trying to shear. However if built the same as mine there are measures taken in the design in the high load areas to prevent this and the cracking may only be superficial.
I have my own view on which one but it's a guess.
An F24 owner that has experienced the same problem should know and therefore be more helpful.
If you are being led astray on the f-boat egroup it is likely that Ian will chime in and set it straight.