Symptoms:
When you connect to vCenter Server using the vSphere Web Client and navigate to Home > Networking and Security, you experience these symptoms:
VMware NSX for vSphere 6.2.x
VMware NSX for vSphere 6.3.x
VMware NSX for vSphere 6.0.x
VMware NSX for vSphere 6.1.x
VMware NSX for vSphere 6.4.x
You can do this in the vSphere Web Client by navigating to Host & Clusters. Select vCenter > Manage > Sessions > Select All Sessions (except "This Session") > Click Terminate Sessions. Logout and log back in from your session. Confirm if NSX Manager is now visible.