Forum Moderators: Robert Charlton & goodroi
Google's message is most certainly automated -- but it does mean that your server returned something problematic. I know you said you validated the file, but how? Have you used the validator or SearchEngineWorld (our sister site)?
Also, note there's a wealth of other information about robots.txt available from that page. Also, though it is an unlikely problem I'd say, check that your server is returning a proper mime type for the .txt file
Content-Type: text/plain