[atlas] "Invalid group id" for Restful API?

I'm trying to create a measurement using Atlas' Restful API [1]. Before that, I created an API key for measurement creation. Using my API key, when executing the simple example shown in [1], the server tells me:

{"error": {"code": 104, "message": "group: Invalid group id"}}

I get the same error code with different JSON-formatted queries. Any ideas what's going on here? [1] https://atlas.ripe.net/docs/measurement-creation-api/ Cheers, Philipp