[CRASH] Team Builder Crashes

Three times in the past day or so, I've been in Team Builder, forming a team, and suddenly I get an hourglass, which doesn't go away. The program doesn't respond, and clicking the window brings up the system message asking me whether to close it or wait. But waiting doesn't seem to help.

Each time, I started the team myself. I don't know if that's a factor, because I always build my own team.

To reproduce:

1. Start League
2. Start Team Builder Party
3. Choose a champ (I happened to be picking jungle, either Zac or Yi each time)
4. Invite friends or don't. It's happened both ways
5. Start to search for pickup players. In most of the crashes, this is where I was when it suddenly hung.

I'd rebooted the computer before the latest crash.

Actually, it's worth note that the reason I rebooted it is that League crashed my entire system during a game yesterday.

I don't know whether I can reproduce this at will. I've also been playing blind pick (which I use to practice new champs), and it never happens there. I do think I have played a couple of TB matches since the first crash, but they may all have been blind pick.

My system:

Windows 10, quad core AMD processor, 512GB SSD boot drive with League installed on it directly, GeForce GTX 960 display adapter, 4K monitor.