The other day I wanted to add some additional ESXi hosts to my lab VCF installation but failed due to the following error

Host validation failed, invalid input. Please check logs for more details
You can remove the validation failed host and proceed with commissioning for the remaining hosts.

The mentioned logfile can be found inside the VCF SDDC manager appliance.

To access the logfile you first have to open an SSH session to your appliance and log in with the vcf user and the password you have set during installation.

The logfile for the host operations is stored here:

/var/log/vmware/vcf/operationsmanager/operationsmanager.log

I placed a tail -f on the logfile and retried the host validation set. In the output I found my problem, it turned out I had a typo in my DNS configuration and the host could just not be found.

java.net.UnknownHostException: esx-vcf-wld01.sddc.local: Name or service not known

After fixing DNS the host validation succeeded and the hosts could be added to the SDDC manager.