Hi John,
Could you please try the following:
1. Check Status of Update Manager Service. If it is not running,
start it and then try enabling the plug-in.
2. If the proxy is enabled without the option
to bypass the local addresses, enable that setting in IE, restart VIClient and
try enabling Update Manager.
3. Try restarting the Update Manager and DB services.
Also, if you could send us the logs, it would be really helpful.
Thanks,
Roli