Spring data elasticsearch repository.count() returns "Current token (VALUE_NUMBER_INT) not of boolean type"

I'm in the process of upgrading a fairly old app from spring boot 2.0.2 to the latest one.
An interim goal was to update to the latest version 2 (2.7.18)
So this is probably a dependency version problem more than anything else, because before update it worked flawlessly.

Now, what the problem is?

[type=json_parse_exception, reason=Current token (VALUE_NUMBER_INT) not of boolean type

full exception

RestStatusException{status=500} org.springframework.data.elasticsearch.RestStatusException: Elasticsearch exception [type=json_parse_exception, reason=Current token (VALUE_NUMBER_INT) not of boolean type
 at [Source: org.elasticsearch.transport.netty4.ByteBufStreamInput@79b99ce2; line: 1, column: 137]]; nested exception is ElasticsearchStatusException[Elasticsearch exception [type=json_parse_exception, reason=Current token (VALUE_NUMBER_INT) not of boolean type

The code that throws is this (simplified):

@Repository
public interface MetadataWithIdRepository extends ElasticsearchRepository<MetadataWithId, String>
(...)
private final MetadataWithIdRepository repository
(...)
repository.count() //throws here

Helpful facts:

1. The problem shows up in integration tests when I spin up an embedded elastic container

• using org.testcontainers.elasticsearch.ElasticsearchContainer - version 1.19.3
• image name: docker.elastic.co/elasticsearch/elasticsearch:6.5.1 (I know it's old, but it has been working fine with the previous spring boot version)

2. used versions

    | spring boot | spring | spring-cloud | boot-data-elastic | elastic-lib | elastic-instance | active mq
old | 2.0.2       | 5.0.6  | Finchley.SR1 | 3.0.7             | 5.6.9       | 6.5.1            |
new | 2.7.18      | 5.3.31 | 2021.0.3     | 4.4.18            | 7.17.15     | 6.5.1            | 5.16.7

3. When I stop my tests with a debugger and query my index I get this:

GET localhost:56229/_cat/indices
yellow open document-domain FQBoF0knRWa-4CVcgAKWsw 5 1 0 0 1.1kb 1.1kb

GET localhost:56229/document-domain/_search
{
    "took": 39,
    "timed_out": false,
    "_shards": {
        "total": 5,
        "successful": 5,
        "skipped": 0,
        "failed": 0
    },
    "hits": {
        "total": 0,
        "max_score": null,
        "hits": []
    }
}

So the instance is up and running.

I tried debugging this in 10 different ways, but every time I hit a dead end.

The only somewhat similar mention of this in english-speaking internets I could find is this, but it does not help

I know that's it's a weird problem and that I have no reproducible example, but maybe someone has seem something similar. I'll be grateful for any hints on what to do with that.

elastic library 7.x is not backwards compatible with elastic service 6.x
upgrading the service will make the problem go away