SQL query cannot extract SQL indices

Hi,

I am trying to perform a simple query using the Elasticsearch query API in Elasticsearch 7.7.0:

POST /_sql?format=txt

{
    "query": "SELECT my_field FROM my_index"
}

I am getting the following error:

{
    "error": {
        "root_cause": [
            {
                "type": "request_seems_to_be_invalid",
                "reason": "Request 'GenericClassTag' cannot be handled; [msg: Cannot extract SQL indices from org.elasticsearch.xpack.sql.action.SqlQueryRequest]"
            }
        ],
        "type": "request_seems_to_be_invalid",
        "reason": "Request 'GenericClassTag' cannot be handled; [msg: Cannot extract SQL indices from org.elasticsearch.xpack.sql.action.SqlQueryRequest]"
    },
    "status": 500
}

I tried the same query using a docker container and it worked. Maybe I have some misconfiguration in my Elasticsearch cluster.
Does anyone have any hints on how to solve this?

Best regards,
Miguel

Those messages look very atypical for Elasticsearch/SQL, I'd say even Elasticsearch (request_seems_to_be_invalid??). Has there been an additional plugin installed, possibly? Or a proxy reinterpreting the replies?

I will try to check with our cluster admins.
Thanks.