Athena Query: Encountered INTERNAL_ERROR_QUERY_ENGINE Error

Hi,

I am creating a table by UNION ALL 6 views with same schema. However, I am getting INTERNAL_ERROR_QUERY_ENGINE when executing the query. I tried to run just the SELECT parts without the CREATE TABLE AS statement but receiving the same error code.

Without UNION ALL I was able to query each table separately, but once I wanted to union them, the error showed up.

The full error message is:

[ErrorCode: INTERNAL_ERROR_QUERY_ENGINE] Amazon Athena experienced an internal error while executing this query. Please contact AWS support for further assistance. You will not be charged for this query. We apologize for the inconvenience.
This query ran against the "<db_name>" database, unless qualified by the query. Please post the error message on our forum  or contact customer support  with Query Id: b8953978-21e8-465f-b0af-857a656a4858

What could be the cause of this error, and how can I fix it? Thanks.