The logs are mysteriously truncated. It might be that Kibana is running out of memory, if you're running Kibana in docker you should see the stack trace, but I'm not sure how this works with docker-compose. I'd suggest running Kibana without docker compose to see if you can't see the logs. Without logs anything we do is just guessing.
I missed this previously:
The amount of total data is not more than 250MB
By default Kibana will read 1000 documents during a migration and try to write all 1000 of them using a bulk index. In your case this will create a bulk indexing payload of at least 250MB which will run into Elasticsearch's
http.max_content_length limit which defaults to 100MB.
You can try to set
migrations.batchSize: 1 to see if migrations would pass.
If each of your 19 saved objects are 250/19=13MB that would be fine, but if one of them are larger than 100MB this would still fail. So you might have to trim back your jupyter notebooks before importing them into Kibana.