ComboBox.ClickItem Asynchronous object call failed

Can you try using VAR instead of LET? Or stripping everything down to the TestComplete example [reference] using aqUtils.Delay(100):

// Calls method asynchronously
var callResult = Runner.CallObjectMethodAsync(list, 'ClickItem', itemText);
 
// Performs the desired user actions
// ...
 
// Waits until the asynchronous call is over
while (! callResult.Completed )
  aqUtils.Delay(100);
 
// Checks the results
MethodResult = callResult.ReturnValue;