The log messages in comment #2 make this sound like a bug in the Xorg modeset driver. To verify that please try logging in to 'Ubuntu on Wayland' instead.
** Package changed: xorg (Ubuntu) => xorg-server (Ubuntu) ** Tags added: multimonitor -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1871997 Title: The primary display will cause black screen when external 4K monitor is configured to flipped To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/xorg-server/+bug/1871997/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
