It's impossible for me to get into a game due to the firewall error during the loading screen and I've tried everything that's suggested. nothing works.
-Google'd the error and look for a fix
-Uninstall/reinstall
-Forward my ports
-disabled all firewalls
-power cycled my router
-direct connected to my modem
-Changed my computer's IP through my router

finally i came to the forum looking to see if this was a common problem. Turns out it was, but not only does no one have a solution yet, No one on the staff has acknowledged the problem in all the threads I've seen and it's annoying not being able to play plus waiting a week with no word that anyone is trying to fix what's wrong.

Big Bump for this.
It happens to me at a lesser extent. I can actually get into the game, however the firewall prompt always comes up first.

This has been happening ever since the Varus patch (no issues before then).
I've also Google'd ways to fix it but nothing has worked (changing firewall/antivirus settings, port forwarding, repairing through the Launcher etc.)

The one thing you can do is wait for the first firewall error prompt, wait about 7 seconds and then click the Retry button. This avoids the second firewall error coming up. However the fact that it even comes up once (delays you about 30 seconds to the loading screen) is an annoyance.

Recently (since Rengar patch), it has gotten even worse in that the whole screen goes black after champ select and alt-tabbing is not possible. The first firewall prompt still comes up after 20-40 seconds, but until it actually gets to the loading screen, the computer is stuck with a black screen.

Before (Varus Patch -> Rengar Patch), it remained at the LoL splash screen, which would appear above anything else you had open, covering up a tiny amount of space in the middle of the screen. However you could still use the computer until the prompt came up.

Some wrenchmen have tried to help (in other threads) but understandably, they haven't been able to offer anything beyond general connection based advice.
It would be great if a Red could reply to this and acknowledge that this is a problem they are aware / not aware of (whatever the case is).

EDIT: A screenshot showing the Firewall Prompt I've been talking about.

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same thing happened to me

I've been having the same issue since Rengar patch as well, but the only problem I have is that once the firewall error pops up, it instantly vanishes behind the "League of Legends" logo. Meaning I have to kill the client.

I have this exact same problem, does anyone know whats going on? I have played for over a year on the same computer, with the same Firewall, anti virus, internet provider everything is constant except that now I cant get into a single game.

What antivirus do you guys all have? If you have the same one, it might have updated recently and changed something related to program access.

BUMP...hoping to find a fix as I too have the exact same issues as OP and have attempted everything.

For those who the message banish behind the logo press Alt+R to retry without clicking because if you click or try alt tabbing the message will banish

Ps: I sent a ticket to riot 4 days ago, i got an answer yesterday they asked for a few stuff, but still no solution if anything i will post it