I tested it in 7.13.1 in a simple setup with an Overview and NewEdit page. When creating a new object, I get the same error. When editing an object with a date, it works as you'd expect when I change the date.
Therefor Maarten is right: your attribute value is empty. You could change the condition to:
$currentObject/Date != empty
and
$currentObject/Date > [%EndOfCurrentWeek%]
or in case you want to show it when there's no value:
$currentObject/Date = empty
or
$currentObject/Date > [%EndOfCurrentWeek%]
This way the first part already determines the visibility before it gives the error.
I have tried this is Mx 7.16 but it works there, the expression i used looks as follows:
$currentObject/date < [%EndOfCurrentWeek%]
Is it possible your object date value is empty? Otherwise maybe try and upgrade your Mx version?