I note that this FW release has a new network card driver. I assume this is to attempt to resolve the network card failures when under high IO load.
We mitigated this issue several months ago on both our Ready Nas Pros by using flow control. However we had our first network card failure in a few months within a week of upgrading to 4.2.20[t7]. This could be resolved by changing the IP address in Frontview and then changing it back to to the original IP address.