I can’t use in-app purchase: This is not a test user account

The problem seems to be caused by an app called AppSync on jailbroken devices.
To enable the in-app purchase again, follow these steps:

1. Uninstall AppSync.
2. Uninstall the app that uses in-app purchase.
3. Reboot your device.
4. Re-download the app from the appstore.
5. Make the in-app purchase.
6. If you want to, reinstall AppSync.

My app crashes during launch

The problem is caused by low memory. Please reboot (power off) your device.