Invoking Callout Activity caused System Exception occured

Hi there. I was planning to show some tooltip messages above the system date time Tray. I used Tooltip Activity first but nothing appeared. Then I changed to Callout Activity. It was fine and I can see the tooltip message once I ran my robot. But I added 6 more Callout Activities and strange things happened. The first Callout message was all right. Somehow the following Callout messages would cause a System Exception, but you will never know which one would cause it. Maybe the second one, maybe the fifth one and sometimes all of the 7 Callouts were OK. Any ideas? Thanks!