So the problem seems to toggle by using that SATA/IDE adapter or not to.
Does anyone have a clue why this device on the IDE bus can interfere
with the EtherNAT? Can this be fixed?
Hard to say whether it's the same cause but I did have problems with EtherNAT + IDE -- if my CT60 was overclocked, in that case I started to see corrupted data transfer (on the IDE channel,not via network) and various random strange bugs. As soon as I used 66 MHz clock, it worked again reliably.