Can't use pre game or post game chat

Id say if you have a weird firewall/router parameters. When did it start? Your only issue is that one and everything is fine in friends list and during game? Could you deactivate your firewall for a game, temporarily and start a custom game with a friend to see what happens. Also try this: https://support-leagueoflegends.riotgames.com/hc/en-us/articles/224826367-Automated-Troubleshooting-Hextech-Repair-Tool

https://support-leagueoflegends.riotgames.com/hc/en-us/articles/201752664-Troubleshooting-Connection-Issues "port forwarding"

Let us know how it goes.