Explore datasource elasticsearch getting error

Hi there!
When constructing a query from a valid Elasticsearch datasource, with unique count for a term, grouped by another valid / existing term (keyword) and then a date histogram, an “Unkown elastic error response” error is returned. When pressing the “Split” button, however, the result is rendered on the “splited” screen and the original screen remains in error.!
Is this a known error or may I doing something wrong?
We are using Grafana v6.6.2 (3fa63cfc34)
Thanks in advance.