Fix Node.js 500 Error

Our production-level Node.js app has suddenly begun returning an HTTP 500 Internal Server Error whenever a REST POST request hits one of the API endpoints. All other request types appear stable, so I’m convinced the fault sits somewhere in the endpoint logic rather than in server configuration or the database layer.

What I need you to do is jump straight into the codebase, trace the exact break point, and deliver a clean, documented fix that brings every POST route back to life. Express.js is the framework in play, alongside typical middleware for authentication and database access, so familiarity there will speed things up. Robust debugging, readable commits, and regression tests are all non-negotiable because the patch will go directly into production once it passes QA.

Deliverables
• Detailed diagnosis of the root cause
• Patch or refactor that eliminates the 500 error
• Confirmation that all POST calls return the correct 2xx responses (manual and automated tests)
• Brief hand-off note summarising the fix and any follow-up recommendations

If you have resolved similar 500 issues before, especially on REST architectures, share a quick example and your expected turnaround so we can get started immediately.