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.