When DSpace XMLUI is installed into Tomcat in root context (that is, as webapps/ROOT), the administrative page URLs conflict with the Tomcat Administrator app which uses the same namespace. The Administrator may not have been installed, and is no longer supplied in Tomcat v6, so one site will have the problem and another never see it.
Klaus Hessellund reports that renaming the Tomcat Administrator (to e.g. webapps/tomcatadministrator) corrected the problem.
If you have the Administrator installed but don't use it, you could simply delete it.
The problem is not seen in JSPUI because the corresponding pages are named