Please try signing out of your Rovio Account, then connect to Facebook. If Facebook connects, you should then be able to sign in to your Rovio Account to continue playing.
If that doesn't work, please try the following:
- Sign out of your Rovio Account and close Angry Birds Epic
- Sign in to Facebook in a web browser
- Delete Epic from Facebook by going to Settings > Apps > Click the ‘X’ over Angry Birds Epic > Click 'Remove’
- Open Angry Birds Epic and connect to Facebook again
- Sign in to Rovio Account