[Koha] Z39.50 server finds the records but delivers no fields

Hi all I'm trying to use my koha instance as a Z39.50 server. I uncommented the statements in /etc/koha/sites/_mysite_/koha-conf.xml. The sever is up. I can search the records. They are displayed but without any field-data. Only the LDR is displayed. Where is my mistake? Beda