Hang on splash logo while booting

[Peter Flynn]

The other day I installed Ubuntu 17.04 from USB onto a new Dell XPS 15 
laptop (whole disk, wiping away Windows).

(I wanted 17.10 but was unable to get it to boot from USB because it 
hangs on the Ubuntu logo with the red dots under it. The dots started 
white, and slowly changed to red one by one...but on the last one it 
stayed there and nothing more happened. I waited for 15 mins and then 
hit the power button.)

So I used 17.04 instead and everything went perfectly, using whole-disk 
encryption.

Today I powered it up and typed the encryption key as prompted, but I 
noticed I made a mistake in typing. No problem, it asked me for it 
again, so I did it right and it started turning the white dots red under 
the logo...and then hung on the third dot. The message underneath said

cryptsetup: nvme0n1p5_crypt set up successfully

I powered off and restarted, and picked Advanced Options for Ubuntu from 
the Grub menu, and selected 4.10.0-38-generic (recovery mode). This 
boots in text mode and asks for the disk to be unlocked. I give the 
crypt key and it unlocks OK. I ran Update grub bootloader from the menu, 
and fsck'd the file systems, and then went for resume and got the normal 
login prompt. Everything seems to be working, except...

...when I log out and power off, then restart, I get the same problem: 
the Ubuntu logo and little red dots hang in the same place.

I have no idea what it is doing while it is cycling the red and white 
dots, so I don't know if the problem is the logo itself or something 
executing in the background (grub?).

Is there a way to replace the Ubuntu logo with something else while booting?

///Peter