Mark, if you have a backup the file is here
Code:
[zimbra@office tmp]$ find /opt/zimbra -name displayMessage_tag.class 2>/dev/null
/opt/zimbra/jetty-6.1.5/work/zimbra/jsp/org/apache/jsp/tag/web/message/displayMessage_tag.class
as long as you are on 5.0.4 as between releases could be a different jetty version!