Google issues with robots.txt

When I noticed that Google was unable to read my sitemap anymore since
about a couple of weeks now (without any changes having been made to
my site), I had a look into Google Webmaster Tools and found out that
apparently, Google is now unable to access my robots.txt file. I tried
to access it with “Fetch as Google”, but all I get is the error
message “Missing robots.txt”. When I try to access the file in my
browser by entering the URL manually, everything works fine. Other
search engines such as Yahoo continue to crawl my site without any
problems whatsoever.

To make sure the problem is not located inside of the robots.txt file,
I used Google Webmaster Tools to create a new robots.txt file,
uploaded it into wordpress and replaced the old one with it – with the
same result.

I have WordPress installed on all of my domains, just having issues with 2 of them.

Help?