solr 6.4.0 - Sorry, no dataimport-handler defined

I have installed solr 6.4.0 on ubuntu 16.04 LTS and want to use a configuration previously used on solr CLOUD 5.2.1

After creating a new core and linking the config files I can now see the core on the web interface and the files correctly under the core's administration interface.

The update link is greyed out and upon clicking on it it displays the error message:

Sorry, no dataimport-handler defined!

The things I checked:

1. Solrconfig loads jar:

<lib dir="${solr.install.dir:../../../..}/dist/" regex="solr-dataimporthandler-.*\.jar" /> <lib dir="/usr/share/java/" regex="mysql-connector-java-.*\.jar" />

Seems to work, as it displays an error message upon changing the filename. I therefore assume the mysql connector is loaded

2. Defined dataimport handler:

in solrconfig:

<requestHandler name="/dataimport" class="org.apache.solr.handler.dataimport.DataImportHandler"> <lst name="defaults"> <str name="config">dataimport.xml</str> </lst> </requestHandler>

3. Dataimporthandler itself

the file dataimport.xml is present and shows:

<dataConfig>
<dataSource type="JdbcDataSource"
            driver="com.mysql.jdbc.Driver"
            url="jdbc:mysql://localhost:3306/test"
            user="solr"
            password="secret" 
            convertType="true"
            batchSize="-1"
            encoding="UTF-8" />
 ...

The logfile inside the webadmin does not show any errors. I assume that logging is not configured to display this kind of error.

When I change the password inside dataimport.xml or the db name, nothing different happens. Same error message.

Any ideas on how to narrow down the problem?