I notice if I navigate to https://vcenter.domain.com/sdk I get page cannot be displayed. vCenter 5.x certs are 2048, right? I uninstalled KB2661254 to see if that would make a difference - no.
Checking the servers I see:
vCenter:
The service is not working properly.
Certificate is untrusted but the thumbprint for the certificate is accepted.
Composer:
The service is not working properly.
SSL Certificate: Unknown