VIA KM400 /dev/dri/card0 missing

Since a few days, X does not start anymore on my Athlon 2600+ with VIA KM400 integrated graphics running Ubuntu 18.04.

It could have been triggerd by the update of some Ubuntu packages but of course I cannot be sure. What I am sure of is that I changed absolutely nothing about X configuration.

The following packages were updated just before X started to fail:

Start-Date: 2020-11-05  07:58:29
Commandline: apt-get upgrade
Upgrade: libc6-dbg:i386 (2.27-3ubuntu1.2, 2.27-3ubuntu1.3), libc6-dev:i386 (2.27-3ubuntu1.2, 2.27-3ubuntu1.3), libsystemd0:i386 (237-3ubuntu10.42, 237-3ubuntu10.43), libc6:i386 (2.27-3ubuntu1.2, 2.27-3ubuntu1.3), udev:i386 (237-3ubuntu10.42, 237-3ubuntu10.43), locales:i386 (2.27-3ubuntu1.2, 2.27-3ubuntu1.3), libudev1:i386 (237-3ubuntu10.42, 237-3ubuntu10.43), libc-bin:i386 (2.27-3ubuntu1.2, 2.27-3ubuntu1.3), libudev-dev:i386 (237-3ubuntu10.42, 237-3ubuntu10.43), systemd-sysv:i386 (237-3ubuntu10.42, 237-3ubuntu10.43), libpam-systemd:i386 (237-3ubuntu10.42, 237-3ubuntu10.43), systemd:i386 (237-3ubuntu10.42, 237-3ubuntu10.43), libnss-systemd:i386 (237-3ubuntu10.42, 237-3ubuntu10.43), libc-dev-bin:i386 (2.27-3ubuntu1.2, 2.27-3ubuntu1.3), multiarch-support:i386 (2.27-3ubuntu1.2, 2.27-3ubuntu1.3)
End-Date: 2020-11-05  08:01:15

Here is what I found out until now:

/dev/fb0 is created at boot time:

[    1.745867] vesafb: mode is 640x480x32, linelength=2560, pages=0
[    1.745869] vesafb: scrolling: redraw
[    1.745872] vesafb: Truecolor: size=0:8:8:8, shift=0:16:8:0
[    1.745916] vesafb: framebuffer at 0xe4000000, mapped to 0x(ptrval), using 1216k, total 1216k
[    1.746088] fbcon: Deferring console take-over
[    1.746090] fb0: VESA VGA frame buffer device

ls -lrt /dev/fb*
crw-rw---- 1 root video 29, 0 Nov  8 09:49 /dev/fb0

but uvesafb reports error -22:

[ 7223.306413] uvesafb: , , , OEM: VIA KM400
[ 7223.306608] uvesafb: Getting mode info block for mode 0x101 failed (eax=0x14f, err=0)
[ 7223.306775] uvesafb: Getting mode info block for mode 0x102 failed (eax=0x14f, err=0)
[ 7223.306942] uvesafb: Getting mode info block for mode 0x103 failed (eax=0x14f, err=0)
[ 7223.307113] uvesafb: Getting mode info block for mode 0x105 failed (eax=0x14f, err=0)
[ 7223.307286] uvesafb: Getting mode info block for mode 0x107 failed (eax=0x14f, err=0)
[ 7223.307463] uvesafb: Getting mode info block for mode 0x108 failed (eax=0x14f, err=0)
[ 7223.307644] uvesafb: Getting mode info block for mode 0x109 failed (eax=0x14f, err=0)
[ 7223.307830] uvesafb: Getting mode info block for mode 0x10a failed (eax=0x14f, err=0)
[ 7223.308019] uvesafb: Getting mode info block for mode 0x10b failed (eax=0x14f, err=0)
[ 7223.308212] uvesafb: Getting mode info block for mode 0x10c failed (eax=0x14f, err=0)
[ 7223.308409] uvesafb: Getting mode info block for mode 0x111 failed (eax=0x14f, err=0)
[ 7223.308609] uvesafb: Getting mode info block for mode 0x112 failed (eax=0x14f, err=0)
[ 7223.311860] uvesafb: Getting mode info block for mode 0x114 failed (eax=0x14f, err=0)
[ 7223.312090] uvesafb: Getting mode info block for mode 0x115 failed (eax=0x14f, err=0)
[ 7223.314343] uvesafb: Getting mode info block for mode 0x117 failed (eax=0x14f, err=0)
[ 7223.314577] uvesafb: Getting mode info block for mode 0x118 failed (eax=0x14f, err=0)
[ 7223.314799] uvesafb: Getting mode info block for mode 0x11a failed (eax=0x14f, err=0)
[ 7223.315027] uvesafb: Getting mode info block for mode 0x11b failed (eax=0x14f, err=0)
[ 7223.315256] uvesafb: Getting mode info block for mode 0x120 failed (eax=0x14f, err=0)
[ 7223.323299] uvesafb: Getting mode info block for mode 0x122 failed (eax=0x14f, err=0)
[ 7223.323557] uvesafb: Getting mode info block for mode 0x124 failed (eax=0x14f, err=0)
[ 7223.323800] uvesafb: Getting mode info block for mode 0x171 failed (eax=0x14f, err=0)
[ 7223.324045] uvesafb: Getting mode info block for mode 0x173 failed (eax=0x14f, err=0)
[ 7223.324294] uvesafb: Getting mode info block for mode 0x175 failed (eax=0x14f, err=0)
[ 7223.324548] uvesafb: Getting mode info block for mode 0x17c failed (eax=0x14f, err=0)
[ 7223.324805] uvesafb: Getting mode info block for mode 0x17e failed (eax=0x14f, err=0)
[ 7223.333703] uvesafb: Getting mode info block for mode 0x17f failed (eax=0x14f, err=0)
[ 7223.333710] uvesafb: vbe_init() failed with -22
[ 7223.336539] uvesafb: probe of uvesafb.0 failed with error -22

yet the uvesafb module is loaded and v86d is running:

lsmod | grep uvesafb
uvesafb                28672  0

ps aux | grep v86d
root       186  0.0  0.0   3568   932 ?        Ss   09:49   0:00 /sbin/v86d

but no /dev/dri/card0 is created:

ls -lrt /dev/dri*
ls: cannot access '/dev/dri*': No such file or directory

X keeps crashing when activated by lighdm. On all my other computers with a working X there is a /dev/dri/card0 file.

Any idea of what could cause the uvesafb failure to properly recognize the video modes? Could it be related to grub?

Thanks.