Hi @berhard,
If you have access to the database, you can execute an SQL command like
DELETE FROM datavalue where dataelementid = (SELECT dataelementid from dataelement where uid = ‘XXXXXXXX’);
DELETE FROM datavalueaudit where dataelementid = (SELECT dataelementid from dataelement where uid = ‘XXXXXXXX’);
where XXXXXXXX is the UID of the data element you wish to delete.
As from: Can't delete some data elements.
See also thread on: Unable to delete an ou.
Best,
James.