Occasionally a Security Alert popup window will appear, but it'll be missing the action button to accept the "What do you want to do?" As a result, the window cannot be responded to and all subsequent network accesses are blocked.
I've attached an example of one. They always appear in black and white.
The only way I found to get around it, because trying to close it from the task bar results in no action, is to log off and then log back in.