There was a comment in ActionBar PO which still seems to be relevant:

 // Make sure Vaadin has completed its job as part of whatever code is preceding the call to hitAction(..).
 // Chaining hitAction(..) itself often results in failures, due to Vaadin client-server communication
 // being still active (and thus the DOM not yet ready) as a result of the preceding action execution.

https://git.magnolia-cms.com/projects/PLATFORM/repos/test-framework/commits/907726a6f2999232467c564824708c1dfde13cf0

As latest random failures in CE builds showed this could be intermittent, but chaining hitActions could fail predictably like here in Tasks app:
https://git.magnolia-cms.com/projects/PLATFORM/repos/dx-core/pull-requests/130/overview

So far investigation:

• element in ActionBar#hitAction method is found and clicked, checked by javascript:

  ((VaadinWebDriverWrapper) driver).executeScript("var ele = arguments[0];ele.addEventListener('click', function() {ele.setAttribute('automationTrack','true');});", action);
  ...
  -> action.getAttribute("automationTrack")

• situation is the same with:

  action.sendKeys(Keys.ENTER);
  

  or

  ((VaadinWebDriverWrapper) driver).executeScript("arguments[0].click();", action);
  

• Thread.sleep(5000) at the bottom of this method don't help (but actually might help if used at the test level between hitAction invocations .. weird?)