The shaders implementation is riemiers shadow maps, which can be found here http://www.riemers.net/eng/Tutorials/XNA/Csharp/Series3/Shadow_map.php
but I have a very very large engine posting it here is not feasible : - D
Maybe a hint like check your graphics setting, null the graphics texture somewhere or maybe you should use this kind of Blend state or at end of rendering change your state something state to this or that ^_^y.
But most likely your right Jagg, I have mess on some part of my engine that cause the artifacts and probably not the custom effect since it's almost identical to remier's implementation.
I'll dig into my engine thoroughly and find the causes ^_^y Thanks.