API -> Upload files via custom widget and if the file upload hits the 403 error sometimes

What's the full HTTP response in this situation?

I've updated the full http response. Could you please check and share your inputs.

Firstly: this is a peer-to-peer support forum.  If you post a message or a reply then there is no SLA on when you get a response.  It is not good etiquette to chance for one immediately after you post something; someone will reply when they get a chance to do so.  If you need an SLA then you can, of course, contact your partner manager or account manager for alternate, paid, consultancy options.

Secondly: I don't see what I asked for.

Thirdly: I will now be away from the office for a couple of days, so please do not have any expectations that I will - personally - respond to any further messages for the next week or so.  If you can provide answers to the actual question then hopefully someone else can help.