This error is caused because the vSphere Lifecycle Manager service running on port 8084 inside vCenter did not respond within the expected time, resulting in a socket timeout. The underlying reason is usually hung, overloaded, or slow LCM backend service, which prevents the UI from retrieving data and leads the generic " An unexpected error has occurred"
Resolution
Option 1: Using vCenter Server Management : For VMware vCenter Server Lifecycle Manager:
Login to vCenter Server Management using https://<vcenter fqdn>:5480
Click on the services
Select VMware vCenter Server Lifecycle Manager service
Click on Restart
For VMware vSphere Update Manager service :
Select VMware vSphere Update Manager service
Click on Restart
Option 2: Using command-line from vCenter Server
Log in to the vCenter Server Appliance through SSH.