I tried reinstall and adding files slowly from scratch before posting.
Seems there is an upper limit on the number of definitions that H2N will run with, and it's possibly below the 65k limit implied by the error message.
Alternative is that a bad stat is causing the crash (rather than volume of stats). Or it could be volume of stats within an individual folder rather than overall volume of stats.
In the past my mind would have boggled at the thought of one stat crashing the entire application, but I've already managed proof of concept for other similar bugs where one small change can destroy the entire application.