The problem is the definition of a^(1/3) of being the complex solution of the equation x=a^3 which has the smallest argument.
While 8^(1/3) is 2 (agrument = 0) we do NOT get (-8)^(1/3)=-2 (argument = pi) but a complex value with magnitude 2 and argument pi/3.
So whenever y(x) gets neagtive your calculations returns a non-real value and odesolve fails.
Resist from writing |y(x)|^(1/3) as this will work (its even faster), but give you a wrong result (see attachment).
Fortunately there is a compromise built into Mathcad. The root symbol defaults to the real value and that solves your problem (calculation time is significantly higher - not sure why).
