What is the version of the flash player(plugin) you are
Could you also let us know if you are using the standalone or
the plugin version of Flex Builder? If it's plugin, could you let
us know what is your JVM version is?
My story is something like this: I installed NOD32 because I
had problems with a virus attaking my Yahoo Messenger and after
doing a full scan of my hard drives I get same pb as you guys.
First it's working to do my debugging. When I close the debugging
session and start it again the error occurs. I have to mention
another thing that I just remembered: when I started first time
Flex after all the scanning, Windows asked me if I want to unblock
the connection! - maybe this is it. I hope someone will see this
and post here the solution!