Getting "User could not be found (QUERY-Dfbg2)" upon session creation for existing user

{
    "checks": {
        "user": {
            "loginName": "theusername"
        }
    }
}

{
    "code": 5,
    "message": "User could not be found (QUERY-Dfbg2)",
    "details": [
        {
            "@type": "type.googleapis.com/zitadel.v1.ErrorDetail",
            "id": "QUERY-Dfbg2",
            "message": "User could not be found"
        }
    ]
}