its a bug with the connector between apache and tomcat (i am hoping that
thats what your doing)

there's no fix at the moment, assuming that you have the latest release
builds of the various components. It arises from the Last Modified: header.
You can work around this by stopping the servlet from sending this header -
I dont know why it has worked so far and stopped working - did you have to
restart the server? upgrade something?

chanoch