On the settings page, it definitely says "You are already connected to Twitter. Click the image below to change the account you are connected to." - and the 'Enable Twitter' box is checked right? (best to check the simple stuff first)
If that doesn't work - and you're comfortable with PHP code - then you might want to try and debug the issue yourself (it works perfectly for me, I just tested it just now), then the appropriate place to look is around line 879 in live-blogging.php. You may want to do something like add the following:
in between lines 879 and 880, and then when you post a post, see what the output of that is.