Since the upgrade vom 18.0.37 to 18.0.38 we have seen that on end user computers that are using McAfee Antivirus solution with the "WebAdvisor" add-on for web browsers, the add-on is bocking some JavaScript or css files on the Plesk GUI. For sure there are files affected that control the Google Authenticator prompt after login (the screen stays white with no layout, the input box and buttons are visible in the top left corner, but they cannot be used). Also, the "outgoing mail control" page cannot be loaded and stays white as if it does not finish loading.
Solution: Disable McAfee WebAdvisor browser add-on.
Solution: Disable McAfee WebAdvisor browser add-on.