Changing ActiveDirectory Identity Source to use LDAPS - error

Hello,

I'm in the last step of hardening our domain network to use LDAPS rather than LDAP and the only thing left I need to reconfigure is the vCenter Identity source.

But, when I change the settings to use LDAPS, I get this error:

Check the network settings and make sure you have network access to the identity source.

Where can I look (logs etc) to see why this is failing?

I've searched high and low but can't find any solution to this and have updated vcenter to latest version (6.7.0.42100)