When you click the check box, I'm guessing that the web page itself refreshes. TestComplete, after clicking on a check box, tries to check the state of the check box to make sure it completed successfully. So, what you're experiencing is that TestComplete is trying to find the state of the check box but cannot.
What you might want to do is look into actually setting the checked state of the check box (possibly a property called "Checked" or a method called "SetChecked") which would not necessarily go through the GUI to set the box but would have the same effect on your application.
