Client closes after lobby

After the matchmaking lobby, the client normally closes and then a new window loads up, full screen, showing the loading status of all the players. This one sometimes just doesn't load. So I'll try opening a new instance of League of Legends, and it gives me the alert saying it's already running (even though system processes and applications both show it's not running). I tell it to close whatever is running and open a new one, and the game will load up and ask if I want to reconnect to the match in progress, so I say yes, and the whole thing happens all over again. This happens occasionally, I thought it might've been fixed in the latest patch but I'm currently afk in a game again because it wouldn't load so I figured I should finally send a report.