as per the Release notes of 4.2 there is one known issue on esxi upgrade. i.e.
- When you skip hosts during an ESXi upgrade of a vLCM-enabled workload domain, the upgrade may fail
Due to a known vSphere issue, the ESXi upgrade may fail with a "ConstraintValidationException" error when a host is skipped.
Workaround: Check the vSphere Client and its logs for details on what caused the error. Resolve the issues and retry the upgrade.
- in your case u will have to check your lcm.log and esxiupdate.log
Thanks & Regards,
Mohammed Viquar Ahmed