The 6.4.0 version of Solr has a problem with the Dataimport tab in the admin UI. It will say "Sorry, no dataimport-handler defined" when trying to access that tab.
The root cause of the problem is a change in the /admin/mbeans handler, by
SOLR-9947. The section of the output where defined dataimport handlers are listed was changed from QUERYHANDLER to QUERY.