I just upgrade today from 2.5.x to 2.7.8 and every time we post we don't get any tweets, but we do get this error:
403 Forbidden: The request is understood, but has been refused by Twitter. Possible reasons: too many Tweets, same Tweet submitted twice, Tweet longer than 140 characters. Not an error from WP to Twitter.
I tried disconnecting and reconnecting the Twitter account to no avail. This was working fine up until we did the upgrade (our upgrade method is to unzip the files into the folder rather than using the WP built in upgrade method.)