exception [type=search_phase_execution_exception, reason=all shards failed] when trying full text search

I tried to implement a full text search in spring data elasticsearch, but when a request is sent this exception is raised "Elasticsearch exception [type=search_phase_execution_exception, reason=all shards failed]"

MultiMatchQueryBuilder queryBuilder = QueryBuilders.multiMatchQuery(searchTerm)
                .type(MultiMatchQueryBuilder.Type.CROSS_FIELDS)
                .operator(Operator.AND)
                .field("firstName")
                .field("lastName")
                .field("activeDirectoryUsername")
                .field("employeeNumber")
                .field("personalEmail")
                .field("corporateEmail")
                .field("project");

        SearchSourceBuilder searchSourceBuilder = new SearchSourceBuilder()
                .postFilter(queryBuilder);

        SearchRequest request = new SearchRequest("employee");
        request.source(searchSourceBuilder);

        SearchResponse response = client.search(request, RequestOptions.DEFAULT);

the exception is raised in the last line.