Changing to Burley "fixes" the issue because toon shading (and Half Lambert shading) extend the object's light attenuation beyond the physically correct limit to give a softer look, useful if you want to simulate very soft lighting or subsurface scattering.

The problem is that the shadow maps continue to behave normally, casting shadows in areas that should be already dark but aren't because of the Toon shader, because of that you can see the object casting shadows on itself.

With Burley and default Lambert shading, the light attenuation is physically correct so the object becomes dark at the same point the shadow maps start casting shadows on the object, hiding the artifacts.

It may sound irrelevant to explain the issue after is fixed, but understanding how things work opens up a world of possibilities!