The Dev Team has just recently released beta 3 of RedSn0w for both Mac and Windows to fix an error that caused MobileSubstrate to crash on jailbroken devices. This specific error only seemed to occur when using RedSn0w to jailbreak over an already existing jailbreak.
From the Dev Team blog:
The b3 version of redsn0w fixes a problem where re-running redsn0w over an existing jailbreak would cause MobileSubstrate-based apps to stop running until MS was installed again. Now you can re-run the redsn0w jailbreak step without worrying about that (but still remember to de-select the “Install Cydia” option if it’s already installed).os
You can download the latest RedSn0w beta from our Downloads Page.
( If you haven’t already jailbroken, refer to this guide for an untethered iOS 5.0.1 jailbreak for all pre-A5 devices.)