Hi skyraven,
sorry for my lateness in reply.
Server is still running, but there is really a problem with hw monitoring... got some coincidence here: one RAM DIMM was faulty last day and ESXi doesn't recognized it... this behaviour was no problem with 5.5 or 6.0!
So be careful and have a look at your iLO interface (or configure alert mail / SNMP traps).
Thank you for clarifying and writing the solution with the image builder, one sidenote to this: when using an "old" vCenter server with "old" viClient, there is a GUI Extension from VMware flings which makes this process a little bit easier to do. Another way would be using the new image builder built in vCenter Server 6.5 (service is not running by default I think).
"Burn the ISO"? 
... use .NET console of iLO with Virtual media to mount your ISO 
aisgroup: as I said in my previous post: install your ESXi on a microSD and boot it from this device, this way your VMs are always safe when updating/upgrading/downgrading