I found which is the plugin that make the conflict but I cannot disable it because it is related to my theme, therefore I deactivated that plugin (Blogwings Companion) I set Async Javascript and then I reactivated that plugin.
But I have a question about it though, do the Async Javascript settings remain saved? That is, the plugin remains active? Is there any way to find out? I would not want that when Blogwings Compaion is active, it disable Async Javascript, because in which case I uninstall it (async javascript)