In case this catches someone else.

I have a self-signed cert on my Zimbra server tied to my world facing name ( My BES connects using my internal network name (fred.vm.home) and therefore the certificate is flagged as invalid.

I had to change :

to 1 to allow things to get moving again. 7.1.0, 7.1.1 & 7.1.4 worked ok, so perhaps the default changed in 7.2.0 or the upgrade did something funky with my certificate.