Ubuntu guest's xterm.js works, NoVNC just blinks cursor

[JBB]

I'd like to have the choice between both NoVNC and xterm.js for an Ubuntu 16.04 guest, but for some reason after I configured the serial terminal for xterm.js, NoVNC stopped working. When I start a session with it, I just see a blinking cursor in the top left of the window and I can't connect properly.

NoVNC used to work before I added a serial device. Xterm.js is working fine.

Here's what I have in /etc/default/grub:

GRUB_DEFAULT=0
#GRUB_HIDDEN_TIMEOUT=0
GRUB_HIDDEN_TIMEOUT_QUIET=true
GRUB_TIMEOUT=2
GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
GRUB_CMDLINE_LINUX_DEFAULT="splash quiet"
GRUB_CMDLINE_LINUX="console=tty1 console=ttyS0,115200"
GRUB_SERIAL_COMMAND="serial --speed=115200 --unit=0 --word=8 --parity=no --stop=1"

# Uncomment to disable graphical terminal (grub-pc only)
#GRUB_TERMINAL=console
GRUB_TERMINAL="console serial"

This is the output from dmesg | grep tty:

[    0.000000] Command line: BOOT_IMAGE=/boot/vmlinuz-4.15.0-47-generic root=UUID=d5401f88-082a-4763-a6f6-d5d04f0edc61 ro console=tty1 console=ttyS0,115200 splash quiet vt.handoff=7
[    0.000000] Kernel command line: BOOT_IMAGE=/boot/vmlinuz-4.15.0-47-generic root=UUID=d5401f88-082a-4763-a6f6-d5d04f0edc61 ro console=tty1 console=ttyS0,115200 splash quiet vt.handoff=7
[    0.004000] console [tty1] enabled
[    0.004000] console [ttyS0] enabled
[    2.061516] 00:04: ttyS0 at I/O 0x3f8 (irq = 4, base_baud = 115200) is a 16550A

Does anyone know what might be causing the problem with NoVNC?