Elasticsearch-Connector: can't index with Search-API
Elasticsearch has deprecated the string type in 5.x versions, and is now using text or keyword. (see https://www.elastic.co/guide/en/elasticsearch/reference/5.5/string.html)

In /admin/config/search/search-api/&lt;CLUSTER&gt;/&lt;INDEX&gt;/fields, you may change string by text in ID field. Considering this is an integer -id-, you may choose the type integer.