If you are planning to use the report templates in 6.3's new reporting hold off for now as it has a known bug that will eventually prevent you from editing the report template.
I have also had this issue occur on me when I tried converting the new report into report templates
This issue is very intermittent and I have managed to work a full day on a report template before it stopped accepting changes and other times I have had this issue occur within 5min of creating a new report template.
The errors that you will get will all have the following line in it; “Exception has been thrown by the target of an invocation.”
The workaround suggested by Kaseya support and that I'm currently using is to just make a normal report instead of the template. You can still convert the report later into a template once they have resolved the issue.
I have started using the workaround yesterday and I have had no issues so far.