Chronic Dev Team has updated greenpois0n to RC6.1 by patching it with iBook fix. In past few days, all greenpois0n Jailbreakers, who attempted to open e-books purchased from the iBook store were prompted with an error message: There is a problem with the configuration of your iPhone. Please restore with iTunes and reinstall iBooks. And, if the device was restored from iTunes, it removed the jailbreak.
Why iBooks failed to open, if jailbroken on iOS 4.2.1 with Greenpois0n?
Whenever a user tries to open a iBook, it tries to run unsigned or improperly signed binaries as a test to see if the device is jailbroken. If those binaries run successfully, the device is marked as having been jailbroken and the eBook refuses to open. Now, that Chronic Dev Team has patched greenpois0n, you can now restore your device and run the jailbreak again. Download greenpois0n RC6.1 with iBooks fix and follow the guide postedhere.
Download Greenpois0n RC6.1 Jailbreak for Windows and Mac
It is important to note that iPhone Dev team and iH8sn0w has already ported the iBooks fix into PwnageTool 4.2sn0wbreeze 2.2 respectively. If you want to retain unlock, you can use either of them as it restricts the baseband upgrade.