Unfortunately there is an incompatiblity between the Twitter library that iSub is using and iOS 5. It may cause Twitter posting to simply not work, or may cause a crash when it tries to post the message.
The issue has been fixed for the 3.0.5 update, but for now, if you’re having issues with it, please turn it off in settings.
More info on the new update coming soon. Still ironing out the final show-stopper issues with all the new code.