Cannot click vSphere Client Security Warning dialog buttons

Symptoms

• When logging in with the vSphere Client you see a Security Warning dialog with a message similar to:
  An untrusted SSL certificate is installed on "<hostname>" and secure communication cannot be guaranteed. Depending on your security policy, this issue might not represent a security concern. You may need to install a trusted SSL certificate on your server to prevent this warning from appearing.
  
• You cannot click any of the dialog buttons or move the dialog window.

Resolution

This issue occurs if:

• You have a Message of the Day configured for your vCenter Server.
  
  And
  
• You have Update Manager installed and have not acknowledged the Security Warning for it.
  
  And
  
• You have not acknowledged the Security Warning for your vCenter Server.

This issue occurs because the Update Manager Security Warning is underneath the Security Warning displayed from vCenter Server. Although you cannot view the dialog, it must be canceled before you can proceed.

To cancel the dialog underneath the first Security Warning, press Alt+F4. This cancels the first dialog window (that is not visible) and allows you to acknowledge the Security Warning.

To resolve this issue temporarily, remove the Message of the Day, then add it back after you acknowledge the Security Warnings.

Even you can press the "ESC" Key 

Source:-

http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1031779