If you're stuck and can't work out why a problem is occurring with the HIBP API, when you submit a support ticket it's important to provide information in a fashion such that the issue can be replicated. Often, people are using third party apps or scripts which we can't support so the problem needs to be boiled down to the raw HTTP request. The easiest way to do this is to send either a cURL command or a Powershell command in the ticket with the API key redacted, and include the full response (both headers and body) when run locally in your environment.
With cURL
curl --header "hibp-api-key: [redacted]" -i https://haveibeenpwned.com/api/v3/breachedaccount/account-exists%40hibp-integration-tests.com
And the output:
HTTP/1.1 200 OK
Date: Fri, 20 Jan 2023 21:17:29 GMT
Content-Type: application/json; charset=utf-8
Transfer-Encoding: chunked
Connection: keep-alive
CF-Ray: 78cad64f8d3f32a1-BNE
Cache-Control: no-cache
Expires: -1
Strict-Transport-Security: max-age=31536000; includeSubDomains; preload
Vary: Accept-Encoding
CF-Cache-Status: DYNAMIC
Pragma: no-cache
X-Content-Type-Options: nosniff
Set-Cookie: __cf_bm=zjzU.T7AHUCGMWRY0_rTNMwobzzkzpRzsvwS2HcPl1I-1674249449-0-AfVtZ20x1qabk41TeAzexU8Dtsq1a3YALUOZNMAS+FmvJ3NlehcpwRew1X/OXbCG3W63mJoAZpcyWUNYckYGkR0=; path=/; expires=Fri, 20-Jan-23 21:47:29 GMT; domain=.haveibeenpwned.com; HttpOnly; Secure; SameSite=None
Server: cloudflare
[{"Name":"Adobe"}]
With Powershell
Invoke-WebRequest -Headers @{'hibp-api-key' = '[redacted]'} -URI https://haveibeenpwned.com/api/v3/breachedaccount/account-exists%40hibp-integration-tests.com
And the output:
StatusCode : 200
StatusDescription : OK
Content : [{"Name":"Adobe"}]
RawContent : HTTP/1.1 200 OK
Transfer-Encoding: chunked
Connection: keep-alive
CF-Ray: 78cae607ebcae9b7-BNE
Strict-Transport-Security: max-age=31536000; includeSubDomains; preload
Vary: Accept-Encoding
CF-Ca...
Forms : {}
Headers : {[Transfer-Encoding, chunked], [Connection, keep-alive], [CF-Ray, 78cae607ebcae9b7-BNE],
[Strict-Transport-Security, max-age=31536000; includeSubDomains; preload]...}
Images : {}
InputFields : {}
Links : {}
ParsedHtml : mshtml.HTMLDocumentClass
RawContentLength : 18