UpdateSingleSalesforceObject - Error in the application

This could be due to different reasons, but guess would be that the date time format generated by the Now() function is not in the same syntax that is required by the date field in Sales Cloud.

I would try formatting the date as follows:

%%[
var @Lead_Id, @result, @dateTime

set @dateTime = Replace(FormatDate(Now(),"yyyy-MM-ddT","hh:mm:ss"),' ','')
set @Lead_Id = "00Q3100000Z3HIcAAN"
set @result = UpdateSingleSalesforceObject('Lead',@Lead_Id,'Reminder_Email1_Sent__c',@dateTime)
]%%