InsightVM API returning 401

Since last week we experience 401 errors using the API (v4) to InsightVM (for example https://eu.api.insight.rapid7.com/vm/v4/integration/assets). I tried creating new API keys, but to no avail. The weird part is that using a valid API key results in:

{
“_type”: “com.rapid7.spring.rest.error.resource.ErrorResource”,
“status”: 401,
“localized_message”: null,
“message”: “Access to this resource is not allowed.”,
“throwable”: null
}

…while a random string as X-API-Key results in:

{
“message”: “Unauthorized”
}

All of these request did 100% work before. Does anyone know what can be the case here?