Guys,
Many thanks for your reply. It's much appreciated. Perhaps it was the WP plugin "Anti-Malware from GOTMLS.NET" plugin itself that caused this issue to popup. I've got not enough knowledge, but when I ordered the Anti-Malware plugin to clean this js file, it cleaned it and I thought, problem solved but then it turned out that the MailPoet tinymce didn't work. What said before, replaced MailPoet tinymce.js files and issue solved.
Good to know a little more about the background. Thanks swachhsite!