OK, I managed to get the AJAX plugin to work!
The solution was to remove ALL "Set Request Header" actions, even for "Content-Type".
Now using PostToURL action (with Method="GET"), or RequestURL action (and passing parameters in the url), Work!
What I don't understand is, why did the headers cause the issue??
Even after allowing the Content-Type header on the server, setting Content-Type header using AJAX plugin always causes the CORS issue again.
Header set Access-Control-Allow-Headers "Content-Type"
Thank you!