Basically the issue that you are experiencing is that the PSC Certificate or the vCenter one (Machine SSL) has not been updated correctly on the Lookup Service. That KB is applicable for vSphere 6.x. Only the first step before the resolution mentions vSphere 6.0.
I would suggest to go step by step on the KB reading carefully and in case doubts to raise a support ticket with VMware.
Moderator: Thread moved to the vSphere Upgrade & Install area.
This can be corrected by using the 'lsdoctor' Tool.
In this case you will have to use "python lsdoctor.py -t"
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Thanks heaps, that tool worked like a charm.
After running it the pre-upgrade checks passed and upgraded to 6.7