An html:errors tag does not have a value attribute.
Also you should be using the <%= expr %> tag to evaluate expressions.
If you are wanting to specify the value of a textfield then
<html;text property="duur" value="<%= toets.getDuur() %>"/>
<html:errors property="duur"/>
However the better way in your Action is to set the values in the FormBean, and then forward to the page to display. It should pick up the values from the form bean automatically.
Cheers,
evnafets