I use a short (3ft) standard Cat5 ethernet cable, but it comes from a TRENDnet Powerline adapter plugged into a 110 power outlet (with the other TRENDnet Powerline adapter plugged into my router). The connection is there when there is no hashing - I can reach the S9 on the LAN no problem.

I didn't save the log but IIRC, didn't see anything alarming in it. Had stuff like:

Check Chain[J8] ASIC RT error: (asic index start from 1-63)

but all the chips seemed to be showing as working on Miner Status although the Status of the Pool Connection was showing as Dead. Next time this happens I will save the log...

I will also save the log next time I reboot and it takes a long time.