Black pointed Background

Hello Guys,

i hope you can help. The following problem is, that i use a thrid party plugin. But it feels like, the arcane design pull a black pointed background to some plugins. I can reproduce it even on the mainpage with the youtube background video.

As you can see in the screenshots, in the preview it seems all fine, but in the result on the page it has these strange pointed dots or whatever on it. There is no attitude do deal with.
Do you have any clue what it can be and how i can fix this?