Please make sure that the Agent Manager (FglAM) hosting VMware Performance agent is on the latest version otherwise upgrade it if required.
It is possible this is caused by a 'VMware-ConnServer-Discovery' agent being deactivated or stopped (automatically, and not by the user). This can be solved by restarting the 'VMware-ConnServer-Discovery' agent, or completely removing it. This agent is automatically recreated, but may not always exist for the end user to perform this action.
Note: With VMware cartridge/agent version 5.7.2, a timeout check for the validation check/section has been added that will prevent the validation step spinning for several hours.