You said you cut the slots with a Slot Machine - That's good

You said you drilled a wicking hole - also good...

The question that comes to my mind is: Did you have the rudder flexed MORE THAN IT WOULD TRAVEL IN FLIGHT before adding the CA?

I find that this type of failure is almost always caused by this. You can't just flex the surface a little (either in hopes of keeping a small gap or any other reason), it MUST be flexed farther than the servo will move it or this type of failure is almost sure to happen.