Boot problems with Ubuntu 15.04

I decided to upgrade to the latest Ubuntu 15.04 named Vivid Vervet  and it all seemed to run smoothly. Until I rebooted and ... nothing! After trying many different things, I managed to force it to display an error message indicating that it was waiting for the Plymouth splash screen. This pointed to the root cause of the problem: Plymouth, the boot-time splash screen manager. Since this machine doesn't even have X installed, and has never had a spash screen, and doesn't even have a screen attached (I do everything using ssh), I don't understand why this new version felt it was OK to install something that would block it from booting.

Anyway, here's how to fix this problem:

• If you don't normally use a screen and keyboard, you'll need to attach them.
• Reboot and press and hold the shift key to bring up the Ubuntu GRUB menu
• Select Advanced options and then select the line that ends in (recovery mode). On my machine, it was the third line.
• Press Return and the boot process continues, bringing you to a menu. Select Drop to root shell prompt and a command-line prompt will show on the bottom of the screen.
• At this point, everything is read-only and only the root file system is mounted, so we need to enable these:

mount -o remount,rw /
mount --all

• Now remove the Plymouth boot splash screen:

apt-get remove plymouth-theme-ubuntu-text

• Next restore the usual boot-time progress display by editing /etc/default/grub and changing the line

GRUB_CMDLINE_LINUX_DEFAULT="splash quiet"
to
GRUB_CMDLINE_LINUX_DEFAULT=""

• Finally regenerate the GRUB settings and reboot

update-grub2
reboot

At this point it should all boot correctly.