phranque - 10:42 am on May 19, 2013 (gmt 0)
googlebot does not see the error in the server error log.
you need to find out what response googlebot is getting from the server, starting with the status code.
you can find this in the server access log.
for example, sometimes an "[error]" in the server error log causes a "500" status code, which would not be an interesting response for googlebot.
you might try accessing your robots.txt from a browser with a user agent switcher so that you look like a googlebot request and see if that gives you any clues.