Can you please tried with some other browser, or just uninstall plugin and re-install the same.
might be work.
Things to try:
1. reinstall the plugin
2. Try a different browser
3. verify plugin is enabled on your browser.
One of these three or a combination thereof should work.
how to enable plugin in browser for Firefox and chrome?
in vCenter 5.5, CIP (Client Integration Plug-in) has some issues using Chrome v45. Try different version of web browser i-e IE, Firefox. For further help, plz follow Vladan's blog post https://www.vladan.fr/vsphere-web-client-integration-plug-in-fails/. I hope it will solve your issue...