Resolve Flutter Signup Issue

I need a sharp-eyed Flutter / Node.js troubleshooter to pinpoint and fix an intermittent signup failure in my mobile app. The OTP step completes successfully, yet many users (on both iOS and Android) hit a wall when the form tries to create the account. No explicit error is returned to the UI, but the account never appears in the database.

Here is what I can share right away:
• Front end: Flutter 3.x, using MSG91 for phone-based OTP.
• Back end: Node.js (Express) REST API, MongoDB.
• Flow: user enters phone → receives OTP → OTP verified → form posts profile details → failure occurs after the POST.

What I need from you is a fast, reproducible diagnosis, a clean code fix (whether in Dart, the REST endpoint, or both), and a short hand-off note so my team understands the root cause. Please come with solid debugging habits—Charles/Proxyman, Flutter DevTools, Postman, whatever gets you to a reliable solution—and be ready to test on both platforms before shipping the patch.