Ensure that the Load script DateFormat matches the dimension's current date format.
For example:
If the load script regional default is SET DateFormat='M/D/YYYY';, but the date field (dimension) used has a different formatting (e.g YYYY-MM-DD), configure the variable to say SET DateFormat='YYYY-MM-DD'; then reload the app which should fix the issue. An alternative would be to use the Date() function in the load script to match the default load script date format.