Playback Interruptions, loses Zone

Roon playback regularly interrupts after about 30-45 Minutes. Playback stops and then the zone disappears from the bottom right of the UI. After a few seconds, the zone comes back and I can press play again. I am not getting any error message when this happens.
When using jRiver, Tidal or Qobuz Desktop app, playback does not interrupt.
I am using the following configuration:
NUC7PJYH with dietpi and RoonCore installed via dietpi
NUC6i5SYK with Windows 8.1 and RoonBridge connected to this USB-SPDIF converter. From there to the DAC via SPDIF.
I tried to find the logs on my NUC/RoonCore server as per here, but found the var directory does not contain a roon subdirectory.
Does anyone have any hints where I might find the logs?

Just had a playback interruption with Qobuz. I immediately moved the USB Plug in the converter a bit and the Zone came back. I am suspecting that the USB socket is marginal.