This error is very common for users that play both the Web and Mobile versions of 8 Ball Pool Multiplayer. Our developers are aware of this issue and are looking into it.
In the meantime it is very easy for you to work around this issue here are a few steps:
1. Log in to the game on your Web Browser;
2. Go to the Cue section of the Pool Shop and select the "Owned" tab;
3. Click "Use" on the Beginners Cue
After you close the game you should be able to log in normally to the application on your mobile device.
Miniclip would like to thank Harold for this article Suggestion!