Connection would not work; Zerotier Central showed all devices offline; any use of zerotier-cli produced the “segmentation fault” message.
I have downgraded to 1.4.6, rebooted, all now works
PS - have not downgraded Pi 3’s and Pi4 - they are working OK on 1.6.1
PPS - now upgraded one Pi Zero to 1.6.0 - that works just fine, connects, no error messages with zerotier-cli and Central shows device as online.