Trying to show sales by customers using dates. But the report doesn't run, throws Report Execution Error. Two key dates, customer account created 'after' date and those sales 'after' then on. What might be the issue?

I setup the same using the default report "Sales By Customer" and entering the same criteria as your screenshot. Mine ran. For resolution of your issue, I would suggest going to a new report from the Reports >> Sales menu. Satisfy yourself the new reports runs as a default then progressively add the criteria. Sometimes customized reports get corrupted and I can't explain that other than rebuilding.

Did as explained and it works, thanks for that suggestion. Guess the one that errors somehow got corrupted.