Verify that you are not using any non alpha-numeric passwords, and that no utilities or Windows permissions are preventing the installation from occurring. For more information, see VMware Update Manager Plug-In fails to install (1003468).
In the C:\ProgramData\VMware\VMware Update Manager\Logs\vmware-vum-server-log4cpp.log file on the system running vSphere Update Manager, you see entries similar to:
HostUpdateDepotManager' 5352 ERROR] [scanHost20, 347] XML parsing error for result at host: esxihost_fqdn (entity: esxihostname). Error: Error while parsing metadata string: Class not registered.