Pro-ject Streambox S2 Ultra issue

Hi everybody,
I need help for my a.m. device and I can’t get it from Pro-ject itself because I bought it from a German dealer (on ebay) and I live in Italy. Big mistake, but what is done is done. Since the control SW is a version of Volumio, I try to get help here.
Here’s the problem: as long as I am using the device with its matching Pre-Box S2 Digital everything is fine, but I can’t get it work properly in DSD direct mode with another DAC (Denafrips Ares 2).
The connection works fine up to DSD128; but with DSD256 files the reproduction becomes noisy and intermittent.
Any suggestion?
Thanks and regards.
Massimo