The following skins appear to crash the loading screen for everyone, meaning that no one can connect to the game. Please don't use these skins until Riot resolves this.
* Battle Boss Qiyana
* Battle Boss Yasuo
* Arcade Caitlyn (including Prestige Edition)
* Arcade Kai'Sa
Personally, I get to about 30-80% before the game freezes, and I get a system error saying that League cannot run. Then I have to restart my computer to get it to close. From what I can tell, everyone else in game experiences this as well.