This error can occur when recurring donation has been processed and cancelled on the same day.
When the donation is processed into Salesforce MoveData will set a Stage (
Opportunity.StageName) ofClosed Wonand Close Date (Opportunity.CloseDate) ofTODAYWhen the regular gift is processed into Salesforce MoveData will set a Status (
npe03__Recurring_Donation__c.npsp__Status__c) ofCancelled
Thus, since the Opportunity will have a Close Date of TODAY, and in cancelling the Recurring Donation NPSP will set an End Date (EndDate) of TODAY, the validation rule will trigger (End Date must be later than the last Closed Won Opportunity's Close Date) and prevent the update from saving.
To get around this is to simply reprocess the same notification the following day. NPSP will set an End Date of Today + 1 thereby avoiding the validation error.