Recently we encountered an issue when we tried to deploy VMs from templates it crashed vCenter Server. VM deployment got stuck between 70%-95% and after a few minutes vCenter Web Clients were not responding and we had to restart vCenter services to get it back up and running.
It seems that templates were missing some info – check https://kb.vmware.com/s/article/20565429. After converting templates back to VMs and again back to templates the deployments were successful and vCenter Server no longer crashed.
Update (13.07.2018) – Same issue affects Veritas NetBackup to get an inventory from a vCenter Server – error “Validation constraint violation”. More info – https://www.veritas.com/support/en_US/article.100033934