Comment 4 for bug 1722745

I have this problem again, however this time I updated a bunch of packages and rebooted straight away and now have the problem after boot.

$ sudo systemctl virtualbox restart

Unknown operation virtualbox.

$ ps aux | grep plymouth
root 735 0.0 0.3 139000 28576 ? S 08:37 0:00 @sbin/plymouthd --mode=boot --pid-file=/run/plymouth/pid --attach-to-session
root 1029 0.0 0.0 12936 1124 ? Ss 08:37 0:00 /bin/plymouth --wait
root 1290 0.0 0.0 12936 1076 ? S 08:37 0:00 /bin/plymouth quit
root 1376 0.0 0.0 12936 1128 ? S 08:37 0:00 plymouth --ping
root 1377 0.0 0.0 12936 1016 ? S 08:37 0:00 plymouth --ping
root 1381 0.0 0.0 12936 1040 ? S 08:37 0:00 plymouth --ping

Plymouth is waiting, quiting, pinging.

$ sudo service virtualbox restart is the only thing that works, but you have to kill the latest plymouth --ping it spawns in order for the service to start.

Starting the virtualbox service on boot can sometimes delay the boot up process due to this bug.