Suppose a general question: What does it mean that the Wronskian at a certain point is not even defined? take for example two solutions for a second order ODE: $$ y_1(x)=\frac{1}{x^{3}} ~~~~\text{and} ~~~~y_2(x)=x^{2}~.$$
We get that $W=\frac{5}{x^{2}}$ which is not defined on $x=0$.
What is the correct explanation?
The solution to your differential equation is by definition a differentiable function defined on an interval of existence.
In your example the function $y_1(x)=\frac{1}{x^3}$ is not defined at $x=0$ so at this point the wronskian is not defined.
We need to stay within the interval of existence.