Okay, thank you everyone, the problem is solved for me!
You guys already said how to do, but I want to make it clear. I hope this will help some people who are in trouble.
Here's how you solve this problem, step by step!
BOTH have to do these steps, in the same time, preferably.
- Log out of Facebook on KoT, and completely close the app.
- Delete the Facebook & Facebook Messenger app of your phone (also make sure you're not logged on Facebook with your phone's web browser). Don't worry, you can download back the apps since the steps are done.
- On your Facebook account, go on your authorized applications (settings/applications), and completely delete the KoT authorized app (the Facebook app, not the app on your phone!). This is the most important step, don't miss it! I recommend to do this step on your computer.
- Launch KoT, and log in with your Facebook account. Now, the phone's web browser should launch. Then, enter your email and your password. Connect. It asks you to authorize the KoT app. Do it. Then you're back on your game.
- Now wait few minutes (between 5-10 max), and you should see your friend on your KoT's friendlist (it's better to relaunch the game few times on this step)!
Sorry for my bad english, I hope it is clear and this will work for you too!