Looks like your antivirus thinks the launcher is a virus and clocking it from launching. Just to make sure I would uninstall league completely and reinstall. After reinstall, add the leagur launcher file as an exception in your antivirus. Should work then.

This was taken from reddit, a bunch of other people are getting this

This is a problem with Avast adding it to its quarantine chest for some reason. All you need to do to fix this:
1.) Go to Scan in the Avast interface
2.) Select "Scan for viruses"
3.) Click "Quarantine (Virus Chest)" at the bottom
4.) Right click on "LoLLauncher.exe" and press "Restore and add to exclusions"
This should fix it. Seems to have happened suddenly today but it's an easy fix.

LoL support TRUMPS D2JSP.

talk to this guy if you havent yet
http://forums.d2jsp.org/topic.php?t=73982072&f=133

seems he had same problem and fixed it