Anyone, any idea? Please help.
No clue, never had that happen. On occasion when I try to debug I'll get an error that I need the Debug version of flash player. Seems about every third 'auto update' of flash player loads the non-debug version even though I have the debug version already. To fix it, I just go to the adobe site and download the latest debug version. But even then the debug option is available, just doesn't work.
One possibility though, is if you changed your default browser to Chrome (or other browser that debug doesn't support), and Flash Builder is set to use your default browser for debugging, that might do it.