Yeah, I noticed that the fix posted can't be applied; the file is 'admin-functions.php', though, not 'admin.php' and the developers have moved the damn function off to another php file in the system, obviously.
I've posted about this problem over the past few days to my blog, twice. First, if an official solution doesn't work first time you try it - it didn't for me when I did my first-ever install (2.0), I had to go hunting the web - then don't discount it forever as it may work next time you encounter the error. And believe me, if my experience is anything to go by, you're likely to.
At the end of last week, after I'd run the upgrade from 2.0.2 to 2.0.3, all was well. A few hours later, all was not well. The fix that time round was the firewall privacy settings which hadn't worked the first time for me. So, all well and good. I was able to do everything again.
Today, got up, wrote a post to my blog and the [insert foul language] error was back; not only that, but what's doubly annoying is when you hit 'back' on your browser you've lost what you wrote, which is pretty lame. It would be marginally less irritating if you didn't. I'd say cut and paste before every post but that's an added layer of nuisance usually, and this error seems to pop up randomly.
Today's fix for me, which may or may not last, which may or may not work next time it happens, was to go into options/general, as one person above suggested and not to change the site to wordpress but to simply add a / - so, for me, spicycauldron.com became spicycauldron.com/ and that worked straightaway.
I know WHY 'you need to enable sending referrers' but the problems it seeks to avoid are far, far outweighed by the inconvenience, frustration and anger it evokes in a huge number of WordPress users. Developers, please note: get rid of it altogether. Approach the problem from scratch. Ditch this because it cannot, CANNOT be fixed - you've had plenty of new versions to do so and still haven't - so why not try approaching what you're trying to do from a new perspective altogether. Surely you can provide the security you're seeking to without locking out so many users from their sites? Without official solutions as likely to not work as work? Without users desperately seeking hacks which really WILL compromise the security of their blogs? I know I for one would give anything to see the back of this loathesome error. I absolutely hate it and cuss whoever thought it was a good thing to implement in the first place. Rethink! Please!
The Spicy Cauldron