Has anyone else experienced this or know what might be going on?
Best answer by The Infinite Sum
Is this after you reach the page which asks for the file that you want to upload? The address should be https://soundcloud.com/upload in your web browser. If you're on that page and it's still giving you a pop-up then logging out of your account it could be a browser add-on. Do you have any similar add-ons installed for Chrome and Firefox? Are you on a Windows based machine with Anti-virus software?