I, too, also get the error. I have a long robots.txt file. In the box that shows the contents of your robots.txt, clear half of it and then test it with the various user agents. Then check the other half of the robots.txt file with the user agents.
If there are more then 5000 characters in that box (which only shows what is contained in your robots.txt, all edits are not saved to your robots.txt file) you will also get the error.
Hope that helps, your not alone on that message. Jake