Using https://webhook.site website to send the same request from both Postman and Studio. Do not send sensitive information, use dummy data for actual credentials.
This method will allow you to see if your request look 100% the same when leaving your machine, be it Postman or Studio. If not, then there will be hints on what to fix in Studio.
Going to Postman and disabling automatic redirect in the settings and looking at the output; it would be interesting to learn if there is a redirect involved in the entire request
in postman what exactly you are passing to authentication check is there any differences for logins, and check any firewalls are restricting the UiPath calls.
instead of using variables try with direct values.