Starting Client leads to no signal from video card.

Any time I start the league client, once the full-screen game portion starts to load, my primary monitor stops receiving a signal from the video card (the monitor says "no signal")

If I alt-tab, I get my windows desktop back, but any time I switch to the league client, the monitor loses signal again.

I'm not sure what to try...

• I've updated all the drivers and rebooted.
• I can play other games just fine, full-screen games included.
• I've attempted to use a text editor to update my \League of Legends\Config\game.cfg file to say WindowMode=1 (instead of 0, which it had) - did not change client behaviour.

Searches for this problem have turned up only stuff from 2013 and 2016. Seems like an every-3-years thing? :)

Any help or debugging tips would be very appreciated.