I get this error when I uninstalled a plugin that was used before. It's been that way for 2 months but I ignore it because it still works the way it does.
Recently an error occurred while creating the CSS / JS minify cache: File "/var/www/vhosts/site.com/httpdocs/wp-content/plugins/mini-twitter-feed/minitwitter.css" doesn't exist.
I'm guessing you have to use paid support to get this resolved.