Few WEM agent are not registering to the Cloud WEM Server after the WEM agent upgrade.

Debug logs would have the below entries:

Event -> BaseBrokerClient`1.GetConnectorAuthContext() : Successfully acquired connector authentication context.  Exception -> ConfigurationDataSourcesHelper.CheckAgentBrokerServiceClient() : System.ServiceModel.Security.SecurityNegotiationException : Could not establish trust relationship for the SSL/TLS secure channel with authority '*****.wem.cloud.com'.
Exception -> ConfigurationDataSourcesHelper.CheckAgentBrokerServiceClient() : System.Net.WebException : The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel.
Exception -> ConfigurationDataSourcesHelper.CheckAgentBrokerServiceClient() : System.Security.Authentication.AuthenticationException : The remote certificate is invalid according to the validation procedure.
Exception -> ConfigurationDataSourcesHelper.CheckBrokerSvcConnection() : Broker Svc Check -> Failed
Exception -> AgentServiceHelper.OpenDataConnection() : Service appears to be Offline and no valid local cache found!
Warning -> InternalRun() : Agent is offline. Skip agent cache sync.
Event -> MainServiceHost.StartAgentProxyService() : Agent Proxy service started
Event -> MainServiceHost.StartAgentMessagingService() : Agent Messaging service started
Event -> MainServiceHost.StartCacheSyncInterfaceService() : Agent Cache Synchronization Interface service started
Event -> MainServiceHost.ReportLegacyAgentWcfService() : Using Citrix Cloud messaging service for agent callback.

1. Delete the value of registry: LastKnownSiteId from HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Norskale\Agent Host
2. Restart the Agent service