A possible scenario in which the error occurs:
One defines a MaxValidDays=365 on a product. An order is made for this product with a ValidFrom. After 350 days have passed since the order was placed, the MaxValidDays value is set to 300. Unfortunately this does not work and the error "Value 'ValidUntil' must be less than ..." is thrown.
The UpdatePWOsValidUntil() methdode of AccProduct.Customizer in this case sets dtValidUntil (new ValidUntil) to tomorrow, because the calculated new value for ValidUntil is in the past (dtValidUntil < dtTomorrow).
PersonWantsOrg.Customizer checks in _Check_ValidUntilValue() if the new ValidUntil is greater than the allowed calculated EndDate. Since the EndDate is calculated based on ValidFrom, the ValidUntil (tomorrow) will be greater than dtEnd (today - 50 days).
This is a product defect (34038).
This will be fixed in a future release of the product. If you require this immediately corrected, please contact support for a hotfix referencing the defect ID 34038.