While auto upgrading to 3.9.1 the upgrading process stopped showing a blank page with following error message in the first line:
Matomo encountered an error: Call to undefined method Piwik\Plugin\Manager::getPluginDirectory() (which lead to: Circular dependency detected while trying to resolve entry 'Piwik\Twig')
I called Matomo again (not reloading the page) and Matomo worked fine with the actual version. I don’t know, whether there may be some misfunction in the background. I don’t hope.
Nevertheless I wanted to send you this feedback. Berhaps this may be helpful.