Hello
I also had this error and could not enter the arena.
To solve this error you have to :
1) ensure that your Angry Birds 2 session is linked with your facebook account, in order to keep your progression saved. If not already then just click on the facebook account after you have finished a level on the map.
2) Clear the cache and all data for the Angry Birds 2 application.
3) Launch Angry Birds 2.
4) start playing , your progression is not available at now so you have to play the first levels.
5) WHen you finish one level, it will show you the “connect to facebook” icon, you have to reconnect on your account and then you will find your progression restored, and you can go to the arena again without this buggy BlaBLAHBLAH window.
Fabrice