Error: 89 after wordpress 3.9.1 upgrade

This sounds like a tricky one. I can’t imagine why an upgrade would trigger this.

Previous people on other sites had a problem when they got the authorisation and access tokens the wrong way round.

I would suggest:

1. Clearing all entries from the Rotating Tweets settings
2. Going back to your Twitter application and clicking on ‘manage API keys’ and then ‘regenerate my access token’
3. Carefully copying the API key, API setting, Access Token, Access Token secret into the Rotating Tweets settings

If that doesn’t work, I’ll try to think of something else! We may need to do some debugging (i.e. setting define('WP_DEBUG', true); in wp-config.php).

Martin