How large / complex is your environment?
Do you know the root log in for the hosts?
I'd be tempted to roll out a new 6.5 vCenter appliance and add the hosts into that and start afresh.
Will make the path to upgrading to a supported solution easier in the long run too.
You are using a very old and unsupported version of vCenter server, I suggest to don't waste your time for Tshooting this problem, and upgrade vSphere to at least version 6.5 ...Please mark my comment as the Correct Answer if this solution resolved your problem
turned out to be an out of date certificate