Weird graphic bug?

Hi fellow WoWers.

Since like a week, I have a weird graphical bug when I play WoW (and only Wow). Blue squares appear on the screen (you can see them in the sky on the picture, but may need to zoom) on certains occasions (speeding effect of dynamic flying, smoke from fires, blur from dusk effect,...). Drivers are updated ( RTX 3070), V-sync on-off doesn't change it, particle effects option doesn't either, no VRS...

Any ideas? (sorry for potential spelling mistakes, english not first language)