Arduno 3G Shield MicroStudio4k control

Hi,
I have problems controlling a MicroStudioCamera 4k with the arduino shield. The shield is connected with a serial-to-usb adapter and programmed under linux in c over the virtual comport ttyUSB. Camera is set to 1.
Switching the tally to red or green works, but the camera is not reacting to any control command, e.g. changing the gain. Control-override is set to 1.
Data sent to the shield is:

char data[19]={ 0xdc,0x42,
0x00, // Reg-Addr L
0x21, // Reg-Addr H 0x2100 = OCDATA
0x57, // 'W'
0x0c, // data length=12,
0x00, // 0x00
// Header
0xff, // camera 255=all
0x05, // command data length length
0x00, // command 0:change configuration
0x00, // reserved = 0x00
//Payload
0x01, // category 1
0x0d, // parameter 13 (gain)
0x01, // data type 1: signed byte
0x00, // operation 0: assign value
0x06, // data: 6=6dB
//Padding
0x00, // 0 padding
0x00, // 0 padding
0x00 }; // o padding

OCLENGTH is set to 12 and OCARM toggled to 1.

Reading back the shield registers with a terminal software
shows that everything was transferred ok:

0x1000 (CONTROL): 03 (Override Tally + Override Control set to 1)
0x2000 (OCARM): 00
0x2001 (OCLENGTH): 0c
0x2100 (OCDATA): ff 05 00 00 01 0d 01 00 06 00 00 00

After setting OCARM to 1, reading back OCARM is 0 => data should be sent, but no changes on the camera.

Sending Tally data with OTDATA =1 (or2), OTLENGTH=1 and OTARM=1 works and changes tally color to red (or green).

After reading the arduino .cpp and .h files I have no idea where my fault is.
Other control parameter changes are also ignored. All sbus remote parameters set to --,
Firmware of camera and arduino shield is ok.

Any help is welcome !

Regards,
Rolf