To check this error - go to the Student Profile and their Conduct tab. Click on exclusions and choose the Exclusion shown, click on the edit button and confirm that the dates entered are correct.
Autumn:
<StartDate> must be a valid date between 2019-01-01 and 2019-08-31
Spring:
<StartDate> must be a valid date between 2019-09-01 and 2019-12-31
Summer:
<StartDate> must be a valid date between 2020-01-01 and 2020-04-12