Upgrading VCSA 6.7 to VCSA 8 fail! – “Encountered an internal error”

I was upgrading a customer’s vCenter Server from version 6.7 to 8. While the upgrade is supported, I encountered warnings (but no errors) during the pre-checks in stage 2. I ignored these warnings and proceeded, which resulted in an upgrade error. It is similar to this KB but I have checked DNS again and it was not a DNS problem. […]