I think you meant VSSetConstantBuffer and PSSetConstantBuffer in the last sentence.
I've replaced setting both cbuffer at the same time
ImmediateContext.VertexShader.SetConstantBuffers(new[] { _matrix_constant_buffer, _light_constant_buffer }, 0,2);
with
ImmediateContext.VertexShader.SetConstantBuffer(_matrix_constant_buffer, 0);
ImmediateContext.VertexShader.SetConstantBuffer(_light_constant_buffer, 1);
and also set the cbuffer to the pixel shader
ImmediateContext.PixelShader.SetConstantBuffer(_light_constant_buffer, 1);
I also tried to attach - : register(cbx) to the cbuffer declaration in the shader code.
but the result is the same error message
Alright, I tried the same code with the sharpdx framework with the same ouput..