I think lot of Flex users saw this error coming up so lets see what we have here.

The most annoying thing is that it seems that there are more actual solutions (so the problem can be very frustrating).

There are more solutions to this problem like:
http://kb.adobe.com/selfservice/viewContent.do?externalId=758bf58b&sliceId=2

I found that my problem was the same as in this site:
http://therush.wordpress.com/2008/03/11/resolved-flex-builder-3-debugger-stopped-working/

But it just doesnt make any sense. The solution for them was to change the debugger from “localhost” to 127.0.0.1. (Right click on the flash player and choose debugger) But why? For me setting the new IP (127.0.0.1) for the debugger worked. They are the same I thought.
So I checked the hosts file. I opened:

C:\WINDOWS\system32\drivers\etc\hosts

And it had this in it:
::1 localhost //this is the problem (not sure that ::1 was the IP but it doesnt matter)
127.0.0.1 localhost

So I had 2 localhost defined (one with some strange thing)

I remoded the bad IP (::1) so just the
127.0.0.1 localhost
remain and of course it works now!

I don’t have any idea how (and why) could it happen, that Flex (or any other program) wrote that stupid thing in my host file (I checked for viruses but I didn’t find anything…)

I try sum the possible solutions here:

1. Restart Flex
2. check you host file

If you have some other ideas I will sum them up here.