Yep im running into the exact same issue now. I was getting error 30 when trying to remediate hosts and taking a look at the esxupdate.log I can see stuff like "HostImage: INFO: Installer <class 'vmware.esximage.Installer.BootBankInstaller.BootBankInstaller'> was not initiated - reason: bootbank is invalid: /bootbank is not a directory!" so taking a look then at the vmkernel.log I saw a lot of "Bootbank cannot be found at path '/bootbank'" I am still messing around trying to see if I can clear this up somehow, or if there is a way to see what is causing it.
Hosts:
Apple MacPro 6,1 64GB
ESXi 6.5U1 Build 5969303