If your installation on subdomain gives a 403 blank screen error after upgrade, you have to edit/remove a .htaccess line to make it work.
Hi, this is affecting you if you upgrade from under 4.1.2:
Mautic Community 4.1.2: One small step for Mautic
You can read the warning in the release docs marked with Danger signs.
I didn’t read it either, but now I’m always reading those announcements before I upgrade.