This issue occurs if:
This issue occurs if 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.