I'm using Flex Builder 3 now and it suddenly refuses to start debugging session. I've reinstalled debugger versions of Flash Player: both 9 and 10 versions. Then I've reinstalled Flex Builder itself allowing it to install it's native Flash Player version. And finally, I've tried to use both IE and Firefox as host for player under debug. How on Earth I could diagnose what happened with FB? Thank you.
PS I've turned firewall of.
I had a similar problem and this is how it was solved: http://therush.wordpress.com/2008/03/11/resolved-flex-builder-3-debugger-stopped-working
If this doesn't solve it: Try to see (with netstat -an on win) if flash player is listening to debug connections. If it isn't, its a problem with player. If it is, try to check builder properties to see if it tries to connect to correct port. Try to telnet to said port to see if player responds.