Hi Richard,
Could you please provide us with an application with which the problem can be reproduced? Also, please let us know whether the window in question can be closed manually at the moment the error occurs - I guess, the behavior is caused by the fact that the window is hanging, and therefore, it does not respond to the user input sent by TestComplete (as it is stated in the error message).
BTW, you can also try clicking a close button or sending the Alt + F4 keystroke to the window instead of calling the Close method - this should help if the problem is that the window does not respond to TestComplete's specific action for some reason.
