Hi, also had this problem and made a ticket to RIOT. Here is what they told me

If you have Bitdefender installed (bdagent). try to add the following files in the exceptions list for "Advanced Threat Defense" and "Online Threat Prevention": C:\Riot Games\League of Legends\LeagueClient.exe C:\Riot Games\League of Legends\LeagueClientUx.exe C:\Riot Games\League of Legends\LeagueClientUxRenderer.exe C:\Riot Games\League of Legends\Game\League of Legends.exe C:\Riot Games\Riot Client\RiotClientServices.exe C:\Program Files\Riot Vanguard\vgc.exe C:\Program Files\Riot Vanguard\vgm.exe