I have got into almost every resource on the topic, but still need your help to make this work. I want to directly upload files to my S3 directly from the browser in my Meteor a
The problem can be that you requesting a SignedUrl for a PUT request but you later on POST the file to S3. You should PUT the file to S3 not POST.
SignedUrls only allow GET and PUT right now.
For more info see this answer: upload-file-from-angularjs-directly-to-amazon-s3-using-signed-url