Your question is vague, therefore my answer will also be.

Option 1) I would assume that you are uploading some large file with a combination that your internet connection is not fast and unstable. Maybe some VPN or other network switching. Thats the way how the browser gets network:reset.

Option 2) The upload code breaks, the services quits, and the internal server proxy resets the connection.

Try to replicate the production environment in your local machine using docker and debug it from there.