You can start it normally if you are going to disable all Avast Shields...the downside ofcourse is we are butt naked.Been happening to me also but i cant change my anti virus for i am using it for more than 5 years.Either we wait for avast team to fix it or to wait for the game devs to patch the game.
I disabled everything and still couldn't play

Tried to add the game to scanning exclusions before disabling, didn't work either
EDIT: I don't know if it helps, but I have some log files of after trying to play; they keep repeating this message;
[... lots ant lots of messages then:]
002167.781| ALWAYS| Queued Dradis event to be sent.
002167.781| ALWAYS| rcp-be-lol-rso-auth| Refresh successful.
002167.782| ALWAYS| rcp-be-lol-login| lol-login: Updating access token.
002167.782| ALWAYS| rcp-be-riot-messaging-service| Access token has changed.
002167.782| ALWAYS| rcp-be-lol-rso-auth| Refreshing tokens in 556.4 seconds...
002168.630| ERROR| rcp-be-lol-clash| CLASH_DISABLED
002168.988| ALWAYS| rcp-be-lol-gameflow| Setting gameflow patcher lock to 1560364290490.
002171.162| ALWAYS| rcp-be-lol-gameflow| Setting gameflow patcher lock to 1560364292664.
002176.067| OKAY| rcp-be-lol-lobby| Set ready retry backoff time: 10.000000
002180.075| ALWAYS| rcp-be-lol-gameflow| Setting gameflow patcher lock to 1560364301577.
002181.738| ALWAYS| rcp-be-lol-gameflow| Setting gameflow patcher lock to 1560364303239.
002186.070| OKAY| rcp-be-lol-lobby| Set ready retry backoff time: 10.000000
002196.091| OKAY| rcp-be-lol-lobby| Set ready retry backoff time: 10.000000
002199.013| ALWAYS| rcp-be-lol-gameflow| Setting gameflow patcher lock to 1560364320514.
002201.168| ALWAYS| rcp-be-lol-gameflow| Setting gameflow patcher lock to 1560364322670.
002206.102| OKAY| rcp-be-lol-lobby| Set ready retry backoff time: 10.000000
002210.102| ALWAYS| rcp-be-lol-gameflow| Setting gameflow patcher lock to 1560364331604.
002211.758| ALWAYS| rcp-be-lol-gameflow| Setting gameflow patcher lock to 1560364333260.
002216.121| OKAY| rcp-be-lol-lobby| Set ready retry backoff time: 10.000000
002216.233| ERROR| RemotingServer: Failed to read from socket 127.0.0.1:52333, recv() returned: -1, errno: 10054 An existing connection was forcibly closed by the remote host.
002216.233| ERROR| RemotingServer: Closing socket due to previous error.
002216.235| ERROR| RemotingServer: Failed to read from socket 127.0.0.1:52341, recv() returned: -1, errno: 10054 An existing connection was forcibly closed by the remote host.
002216.235| ERROR| RemotingServer: Closing socket due to previous error.
002216.236| ERROR| RemotingServer: Failed to read from socket 127.0.0.1:52343, recv() returned: -1, errno: 10054 An existing connection was forcibly closed by the remote host.
002216.236| ERROR| RemotingServer: Closing socket due to previous error.
002216.385| ALWAYS| rcp-be-lol-gameflow| Client is no longer running.
[... and lots of messages after these.]
Another log file says:
[...]
000017.498| ALWAYS| CFG| MultiplayerSettings Encryption=true Compression=true Logging=false Wireless=false Redundancy=false Threshold=0 Resends=20
000017.498| ALWAYS| FLOW| SEJ-6671BCAD
000074.543| ALWAYS| CONN| {"messageType":"riot__game_client__connection_info","message_body":"Disconnected","disconnect_delay":15.000000953674316406,"disconnect_timestamp":59.30670166015625}
000074.543| ALWAYS| CONN| Application terminating due to disconnect while loading
000076.109| ALWAYS| Process Force Terminating.