The latest nightly build for Windows, r30966, can't open _any_ RSS feeds.
For example, opening feed://rss.slashdot.org/Slashdot/slashdot gives
Safari can’t open the page “feed://rss.slashdot.org/Slashdot/slashdot”. The error was: “unknown error” (CFURLErrorDomain:305) Please choose Report Bugs to Apple from the Help menu, note the error number, and describe what you did before you saw this message.
This has been present in builds as far back as two weeks, when I first picked up a nightly. It is not present in the normal Safari installation.
It's always reproducible for me, and once every few tries, also causes a crash, though that seems to be an unrelated issue.
I have cleared the entire prefs folder (in Application Data) a few times, and this does not resolve the issue.
Confirmed in Webkit r30966 but only on Windows. There is no problem on Mac OS X
*** This bug has been marked as a duplicate of 16147 ***