After upgrading to the 5.0 kernel, when booting into the notebook with the monitor connected via VGA, the desktop does not complete the boot. It does not finish loading the screen and does not display the login screen - using an HP-ProBook-6450b. If the boot occurs without the monitor connected, it works normally - after connecting the monitor, it works normally.
If you can't even see the login screen (a.k.a. Display Manager), then certainly it is not a Xfce bug. You should report this issue on your distribution bug tracker.