I have not updated any plugin or installed anything new. Since today, there is a strange Popup ad appearing on my Wordpress admin dashboard. The permalinks of all the pages (except homepage) became invalid, but that problem I solved by going to Settings->Permalinks->"Press Save".
I read this blog, which mentions that two infectious plugins are doing the same thing but I have checked my
wp-content/plugins and have not found any strange folder inside it.
Furthermore, on the page Appearance->Widgets, I am not able to change/click any widget.
Screenshot of My current Dashboard:
Update-1: I found one plugin which I have definitely not installed. But I am not sure if this is the cause. The plugin name is stop-ref-spamers but it's not activated. I don't know if a plugin can harm even if it's not activated.
PS: I do have Google Adsense script in one or two posts and a Google Adsense script in Custom HTML widget.