I left a support ticket and the person who responded (who may or may not be a Riot employee; I think they get some outside people who are considered experts to help with their chat responses if I understood correctly) said to disable my antivirus, run Hextech Repair Tool, the usual, and not to have any overlays. But it seems not to just happen when I have an overlay, and I'm pretty sure Malwarebytes isn't causing the problem (I've had it a few times over the years in regular Summoner's Rift play, but **extremely ** infrequently, so it mainly seems to happen in a large minority of games in TFT. Unfortunately, it seems to only affect my ranked games, which means it's either Ranked in particular that's causing the problem, or this particular patch since that's when ranked came out and I started playing it.