Hi Damian,
I think that, when the 'Insert Sales Order' button is clicked by the problematic line, a new page is loaded. Although there is a delay in 10 seconds, this time can be not enough for the page to be completely loaded, and therefore, TestComplete tries to work with the page before it is ready. Call the Page.Wait method after navigating to a new page to make TestComplete wait until the page is completely loaded. You can find more information in the 'Page.Wait' and 'Waiting For Page' help topics.