Fix JS Fetch Runtime Errors

My web app throws runtime errors each time it tries to pull data from the server. I need an experienced JavaScript debugger to dive into the affected module, trace why the fetch/AJAX call is breaking, and deliver a clean, working solution that eliminates these errors without introducing new ones.

You’ll be working directly with the existing codebase (vanilla JS with a light sprinkling of ES6 features). Once the fix is in place, I’d like a brief explanation of what caused the issue and how you resolved it so I can avoid similar pitfalls down the road.

Acceptance criteria:
• No runtime errors during or after any server-side data fetch.
• Normal user interactions remain untouched and fully functional.
• A concise summary of the root cause and implemented fix accompanies the updated files.

The sooner we can get this stabilized the better, so please let me know your availability to begin.