What the docs say:
Official docs (Microsoft Learn, Bing Web Search error codes): the real errors people hit and what they mean. 400 means a parameter is missing or invalid. 401 means the Ocp-Apim-Subscription-Key is missing or wrong, or you sent redundant auth methods at once. 403 means the monthly quota is exceeded, or the authorization is disabled or expired. 410 means you called over plain HTTP instead of HTTPS. 429 means the QPS (queries per second) quota is exceeded.

What works:
When a Bing Web Search call fails, read the status code before touching your code. 401: check the Ocp-Apim-Subscription-Key header value and make sure you are not sending two auth methods at once. 403: check quota usage and whether the key is still active. 410: switch the request URL to https. 429: slow the request rate, you are over the per-second quota. 400: re-check required parameters, something is missing or malformed.