Check out this thread, specifically Jasper's comment and solution. I assume you have the same issue.
Thanks, that definitely helped! I have been able to import the APIs response into Mendix now. However, I'm stuck at another point now. The API responds with an array within an array:
"rows": [
    [
        "value 1",
        123.212,
        true
    ],
    [
        "value 2",
        542,
        false
    ],
    [
        "value 3",
        342,
        false
    ],
    [
        "value 4",
        5452,
        false
    ],
    [
        "value 5",
        32,
        true
    ]
]
I know I should work with the RESTServices.Primitive call to translate arrays into entities, however, that only seems to work properly for an array with a name right before it, not for an array within an array. Any ideas on this?