Weird blending and light building issues

Hey everyone,

I am currently working on a acene and I get some weird stuff on my floor. When I try to blend two materials and paint over a whole quad of the floor it gets black. I can do a workaround where I just put spots of blended texture everywhere but it can´t be the solution of this problem. The second issue comes when I build my lighting. The whole floor just gets black, or really really dark so you can´t see anything on it but I don´t get any errors. No other object I put in the scene has the same problem, so i am very confused.

how can I solve these problems?