When certain VFX displays on-screen, they expand to form multiple layers of transparent, flashing sheets, generally overlapping to obscure most of what's happening on-screen and lagging the game immensely. This only occurs when those VFX appear, and as soon as they leave the screen everything works as intended.
So far I confirmed three sources: Any items that look like the Defiants' active Magitech Lamps, sometimes when a member of the Storm Legion dies (not always; presumably only a certain enemy type triggers this, but I haven't figured out which one either), and the lighting effect generated by some of Gloamwood's Guardian Waykeepers.
Fortunately, the Storm Legions' visuals vanish shortly after they die, so that causes few problems. The Magitech Lamps are much more common and permanent, though.
It most commonly occurs after killing Shadavarian in the Defiants' Ceremony of Attunement chronicle, as a Magitech Lamp spawns next to a table where the major NPCs stand, but can also be found often in Cape Jule (lots of both magitech lamps and storm legion), various Defiant camps or cities (once again, the lamps), and the Guardian camp directly to the east of the Darkening Deeps dungeon (Waykeeper lights).