Found the root cause on this issue I believe. While the installation was still on 3.2, most of my /Device/Server/Linux devices had changed IP subnets. Once I examined the Infrastructure->Networks view, I noticed that all my old device IP's and their associated subnets were still listed, with the details set to "No Device" and "No Interface".
I deleted all the old IP address entries (none of which were visibly associated with an interface) and the problem disappeared. Wish I had checked the network view earlier, but it's of limited use in my case and I rarely look at it.
Even though the transform error is now resolved, I left the threshold on the Ethernet monitoring templates disabled, as the resulting thousands of daily "info" events triggered by it seemed to be useless churn.