AWS S3 with event: Unable to validate the following destination configurations

Hi,

when i deploy my image resizing code with:
sls deploy --stage acc --bucket mybucketname --verbose

I get:
CloudFormation - CREATE_FAILED - Custom::S3 - ImageResizeCustomS31

An error occurred: ImageResizeCustomS31 - Failed to create resource. Unable to validate the following destination configurations

This is my serverless.yml

service: myproject-image-service
custom:
  uploadFolder: uploads/

provider:
  name: aws
  runtime: nodejs10.x
  region: eu-west-1
  iamRoleStatements:
    - Effect: Allow
      Action:
        - s3:*
      Resource: 'arn:aws:s3:::${opt:bucket}/*'

functions:
  imageResize:
    handler: resizeImageHandler.resize
    environment:
      BUCKET: ${opt:bucket}
      UPLOAD_FOLDER: ${self:custom.uploadFolder}
    events:
      - s3:
          bucket: ${opt:bucket}
          event: s3:ObjectCreated:*
          existing: true
          rules:
            - prefix: ${self:custom.uploadFolder}`

Note: when i remove the events part, it works. Seems to be a timing issue? but how do i solve this?