The Dev Team has released beta 3 of RedSn0w for the Mac and PC to fix an error that caused MobileSubstrate to sometimes crash on jailbroken iOS devices. This specific error only seemed to crash MobileSubstrate when the latest iOS 5.0.1 untether in RedSn0w was run over an 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).
Another tip: If RedSn0w tells you that no identifying data was found when it auto-detects your IPSW, then you can pre-select your iOS device’s 5.0.1 IPSW in RedSnow’s Extras >Select IPSW menu.
As always, you can download the latest RedSn0w beta from our Downloads page. This latest beta follows an update from yesterday that fixed the “launchctl” error in Cydia. We have a dedicated RedSn0w page with everything you need to know about the jailbreak tool, and we have tutorials for jailbreaking iOS 5.0.1 for both the Mac and Windows.