Maybe when you do a full reload there are data that don't have a valid date format. I think Alt() function could help you, you can use this function to check if the fild has the format or formats you need, something like this:
Alt(Date(floor(timestamp#(lach.ReportWeek1,'M/D/YYYY h:mm:ss[.fff] TT')),'YYYY-MM-DD'), 'Invalid Date Format')
You should review the syntax in the help menu...
Hope this helps