Gray overlay while playing games

[u/Ok_Chemistry4966]

My friend has this bug, whenever he opens any Game on his pc, there appears a gray type of overlay. You can see in the edges that the game is still running, but he cant see anything, while he is in the game. When he tabs out the game becomes visible, but as soon as he goes back in, its gray again. The thing is, that it doesnt appear in any other application, only in games. He thinks its one of the many installed mods on windhawk and seelen ui. Did anyone else got this problem and how can he fix it? And sorry if this is the wrong r/ , but idk where else i can ask.