InsightVM API returning 401

Since last week we experience 401 errors using the API (v4) to InsightVM (for example I tried creating new API keys, but to no avail. The weird part is that using a valid API key results in:

“_type”: “”,
“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?