Auth results in "Wrong credentials"

I'm setting up an integration between our Personio account and a third-party SaaS management tool (Zluri), which authenticates against Personio using a Client ID and Client Secret generated from a custom integration (Marketplace → Connected Integrations → Create custom integration).

Every auth attempt is rejected with the following response from POST /v1/auth:

{
"success": false,
"error": {
"code": 0,
"message": "Wrong credentials"
}
}

HTTP status code: 410
Response headers include x-ratelimit-limit: 60 (so the call is reaching your v1 auth endpoint).

What is the fix?