The file upload feature is using the Chrome debugger API. That said, we have not seen this message yet. Are you maybe debugging a website while running the upload macro? Or is there a 2nd instance of Kantu running in Chrome browser with the same profile? (You can run many instances of Kantu at once by using different profiles)
Meanwhile there is a 2nd method to upload files: You can use the xtype command to send file name and enter keys directly to the file upload dialog. This even works in Firefox.