This type of email notifications from workbooks which use live connections, are received when queries to the database are not completing, and Tableau Online cancels the job after a 45 minute timeout.
Working with database administrator and investigating connectivity issues with live connections can be a good troubleshooting starting point.
This error message is displayed in the knowledge base article below.
For more details please also refer to the "Troubleshoot Subscriptions" help article below.
Hope this helps!
I went through the links and the below extract lays down two possible reasons:
"The view snapshot in this email could not be properly rendered." If you receive a subscription with this error message, there could be several reasons: Missing credentials: Some views are published with embedded credentials. You may receive the above error if the embedded credentials are now out-of-date, or if the view was republished without the embedded credentials.
Database temporarily down: If the view has a live database connection and the database was temporarily down when the subscription was being generated, you might receive the above error
The thing is - I have checked for the possibility of missing credentials and Database being temporarily down. None of them seem applicable in our case. That's why we have been wondering the reasons to no avail.
If the troubleshooting doesn't seem to apply I think the best thing to do would be to raise a Support Case and we can delve into it a bit further. There may be additional information in the logs that we can check that could point to a cause of failure.