I have a bunch of mods running and when I use the construction kit this is what I always get

assert

file c:\_skyrim\code\tesv\bsshader\shaders\bslightingshaderproperty.cpp

line:806

and I heard somewhere this could be cause by the nexus mod manager and wonder how to fix this issue or what is needed to at least lesson this error!

You can click ignore for that. Assert errors usually come up when bslightingshaderproperty is setup in a way that CK doesn't recognize, but otherwise it's harmless. CBBE is one body mod known to cause asset errors.