Blender 2.8 What is going on here?

Why does more than 1 material cause this problem and how can I fix it? Thanks in advance!

I would recommend that you show the setup of node on that object

so people can take a look at it and figure it out what was the cause.

This is really a bit too little information as @Xelbayria said. So I would say you played with the Light Path Node somehow LOL.

… looks like common P.E.B.C.A.K.