The host upgrades are marked as 'Skipped'.
The 'Remediation Message' section of the SDDC UI error is marked as 'null'
VCF 5.x
If the SDDC is unable to check the health state of the cluster the upgrade will not proceed.
This is by design.
This can occur if the network proxy server is not available
Check the correct the connectivity of the components to the proxy in the environment.
Once remediated, retry the upgrade.