Is the welcome screen the one that asks you to login or create a new gamecenter account? That's an issue people have had with jailbreaks in the past. The typical solution that has worked is:
1. Close all multitasking apps. Open settings app and log out of gamecenter.
2. Open a game that uses GC, make a throwaway account, and log in. Close that app.
3. Open settings app again and log out of that account. Then log in to your original account.
4. Open a game that uses GC and log in with your original account. It should work now!