** UPDATE 18/12/2020 10:45 EET **
We have discovered an issue on Google's end that impacts queries with Custom JSON/CSV or Google Sheets connectors, causing them to fail intermittently.
While we wait for Google to fix the issue, you can work around the problem by enabling "Anyone with the link" sharing on the source file. The issue may also affect email triggers with CSV or TSV attachments when the report does not have "Anyone with the link" sharing on. To fix any issues with emails temporarily, you can either change the sharing settings to "Anyone with the link", or change the attachment type from CSV or TSV (to Excel for example).
To make sure the issue gets the necessary attention from Google, we would like to ask you to please upvote the related thread on Google's issue tracker.
We will continue to look for solutions to get around the issue and update this article as soon as we have more information. Our apologies for any inconvenience caused by this.