Use uielements wait methods WaitForControlExist, WaitForControlEnabled and WaitForControlReady before you click when dealing with popups, or any button. It's likely that your tests are trying to click before the popup "ok" button is ready.
uiControl.WaitForControlExist(20000);
uiControl.WaitForControlEnabled(20000);
uiControl.WaitForControlReady(20000);
mouse.click(uiControl);