DSD files

I have got a Raspberry Pi3 with Digi2 pro. As dac I use a teac ud-501, connected with a spdf coaxial cable.
The music files are contained in a network HD (Zyxel with upnp Twonky).
I have problems with the playback of dsd files (128 dsf): they are heard with pauses.
If I don't use the Digi2 pro and I connect the Pi3 to the dac with the usb cable everything works fine.
Please help me to listen to the dsd files with the digi2 pro.