thanks for the details!
i was able to see in logs the error that you were mentioning, however i did not found anything special about it that could help to discover the reason of the issue. my advice, if you ever see this error again, would be that you let the original template as it is and let us know inmediatly so we have something to debug, in that case you can continue with your reports by cloning the template and use that clone instead for your final reports.