Is that a direct ethernet connection? Aren’t you going through a powered switch or router? Are you using power lan anywhere in your home network? Wireless bridging somewhere?
It is a long shot, some have reported SQ issues with all the above powered ethernet components. Personally I have only ever heard an impact on SQ with power lan. I have never heard any SQ differences with ethernet cables (although I have never tried exotic audiophile ones). There are technical reasons why poorly made ethernet cables may be the wrong spec or impact throughput or not work at all but SQ is unlikely. There is a long thread on that:
Analogue cables are something else. Again it is a long shot but often when I am mucking about plugging things in an out in tight spaces , or adding a new cable or component I put it all back together wrong. Accidentally, reversing the polarity of speaker or amp cables for example. It might be worth double checking.