Siemens S7 Modbus with Noctra Gebhardt ventilation fans

A customer site has 26 Nicotra Gebhardt EC PFP roof mounted ventilation fans. Control and monitoring is to be via Modbus RTU withing a Siemens PLC.
I am sequentially reading data OK from the fans with the MB_MASTER function, mode 0, address 30001, data length 33 (Modbus function 04, read input registers 0 to 32).
The write function to the fans is not working with mode 1, address 40067, data length 1 (Modbus function code 06, write holding register 66).
Anybody used these fans with Siemens Modbus and have any suggestions? I've tried Nicotra Gebhardt support, but no suggestions so far.