I purchased an Arche late last year and also experienced random NO SIG errors on both Mac and PC.
After getting completely fed up, I arranged through my dealer to have the unit sent back to FocalNaim. After a couple of weeks I received a replacement unit. It is still ‘early days’ but so far I have experienced no issues. I have been able stream Tidal through Roon and Audirvana (both in exclusive access/integer mode) on my Mac continuously for hours.
There was no explanation as to what the problem was with the original Arche. Would love to know what the real story is with these.