Core answer
Start here. This is the direct spoken answer to practice first.
Why this question matters
Full-stack debugging is where practical engineering shows. The answer needs to move from user symptom to browser evidence to backend facts without guessing wildly.
I would reproduce the flow with the same user role, data shape, browser, and environment if possible. In the browser I would inspect console errors, network request, status code, request payload, response body, and timing. If the app has a frontend server layer, I would check whether the failure happened there or in the ASP.NET API. Then I would use correlation ids to find backend logs and confirm validation, authorization, dependency, or database errors.