While trying to make an in-app purchase I got the message: "This is not a Test User account. Please create a new account in the Sandbox environment.". This probably occurs only in jailbroken iPhone devices.

In order to solve this problem I performed the following steps:
- deleted AppSync package from Cydia
- rebooted the iPhone device
- uninstalled the application (in my case it was Ultimate Notifier)
- reinstalled the application