Jamie Crone PRO said almost 3 years ago on StimulusJS, Active Storage and DropzoneJS :
  Thanks for replying! I do have 3 errors pop up on the console, first one being 
Access to XMLHttpRequest at 'https://uatmyskiswap.sfo3.digitaloceanspaces.com/mtn1067cgyun59enm7tg5pepje…Signature=9a216bd…' 
from origin 'http://uat.myskiswap.com' has been blocked by CORS policy: 
Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' 
header is present on the requested resource.

Maybe this is the issue or a separate one. The second one is
dropzone_controller.js:29 Uncaught TypeError: Cannot read property 'signed_id' of undefined
    at dropzone_controller.js:29
    at e.callback (activestorage.js:879)
    at e.value (activestorage.js:841)
    at XMLHttpRequest.<anonymous> (activestorage.js:815)

The error above comes from the this.hiddenInput.value = blob.signed_id  below
    dropzone.on("addedfile", file => {
      setTimeout(() => {
        if (file.accepted) {
          const upload = new DirectUpload(file, this.url)
          upload.create((error, blob) => {
            this.hiddenInput = document.createElement("input")
            this.hiddenInput.type = "hidden"
            this.hiddenInput.name = this.inputTarget.name
            this.hiddenInput.value = blob.signed_id 
            this.inputTarget.parentNode.insertBefore(this.hiddenInput, this.inputTarget.nextSibling)
            dropzone.emit("success", file)
            dropzone.emit("complete", file)
          })
        }
      }, 500)


The last error is because the of the error above which just doesn't create 
activestorage.js:823 
PUT https://uatmyskiswap.sfo3.digitaloceanspaces.com/mtn1067cgyun59enm7tg5pepje…Signature=9a216bd… 
net::ERR_FAILED

When I don't use dropzone I don't have this issue uploading files or avatars