Which parties are involved in Nabisy? 

The BLE is the German Federal Office for Agriculture and Food (Bundesanstalt für Landwirtschaft und Ernährung) that hosts the Nabisy system and is responsible for the Nabisy account, the Proof of Sustainability and everything to do with the Nabisy account.  
ISCC as the certification system is responsible for processing the applications for Nabisy and submitting them to the BLE. This is necessary because ISCC is the system in which the system user is certified. In addition, any changes to your Nabisy account information must be made by submitting the amendment application form to ISCC.