Screen not recognizzed on laptop, xrandr goes "Failed to get size of gamma for output default"

Okay, that was a bold move. Took this old Acer 3633WLMi and gave it a SSD and a shiny 18.04.3 in Lubuntu flavor.
According to most Ubuntu Wikis, this specific laptop should run smoothly, no issue reported...
NOPE

Apart from the wifi being dead silent (will address this later on), my current main problem is being stuck in a 640x480 screen resolution!

Of course, I tried to find some forum discussing this issue, but couldn't find any.... which is pretty understandable, this problem being usually caused by Intel cards and this Acer thing boasting a SiS proprietary [censored] chipset.

Now, to the point (and diagnostic):

$ cat /etc/default/grub
[...]
GRUB_DEFAULT=0
GRUB_TIMEOUT_STYLE=hidden
GRUB_TIMEOUT=0
GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
GRUB_CMDLINE_LINUX=""
[...]

$ dmesg |grep VGA
[    0.454387] pci 0000:01:00.0: vgaarb: setting as boot VGA device
[    0.454391] pci 0000:01:00.0: vgaarb: VGA device added: decodes=io+mem,owns=io+mem,locks=none
[    2.360257] fb0: VESA VGA frame buffer device

$ grep driver /var/log/Xorg.0.log
[    11.433]    X.Org XInput driver : 24.1
[    11.532] (==) Matched sis as autoconfigured driver 0
[    11.532] (==) Matched modesetting as autoconfigured driver 1
[    11.533] (==) Matched fbdev as autoconfigured driver 2
[    11.533] (==) Matched vesa as autoconfigured driver 3
[    11.533] (==) Assigned the driver to the xf86ConfigLayout
[    11.539] (II) Loading /usr/lib/xorg/modules/drivers/modesetting_drv.so
[    11.551] (II) Loading /usr/lib/xorg/modules/drivers/fbdev_drv.so
[    11.560] (II) Loading /usr/lib/xorg/modules/drivers/vesa_drv.so
[    11.562] (==) Matched sis as autoconfigured driver 0
[    11.562] (==) Matched modesetting as autoconfigured driver 1
[    11.562] (==) Matched fbdev as autoconfigured driver 2
[    11.562] (==) Matched vesa as autoconfigured driver 3
[    11.562] (==) Assigned the driver to the xf86ConfigLayout
[    11.563] (II) Loading /usr/lib/xorg/modules/drivers/modesetting_drv.so
[    11.570] (II) Loading /usr/lib/xorg/modules/drivers/fbdev_drv.so
[    11.571] (II) Loading /usr/lib/xorg/modules/drivers/vesa_drv.so
[    11.571] (II) FBDEV: driver for framebuffer: fbdev
[    11.579] (II) VESA: driver for VESA chipsets: vesa
[    12.803]    ABI class: X.Org XInput driver, version 24.1
[    12.803] (II) Using input driver 'libinput' for 'Power Button'
[    12.955] (II) Using input driver 'libinput' for 'Power Button'
[    12.973] (II) No input driver specified, ignoring this device.
[    12.978] (II) Using input driver 'libinput' for 'Sleep Button'
[    13.243] (II) No input driver specified, ignoring this device.
[    13.245] (II) Using input driver 'libinput' for 'Sleep Button'
[    13.258] (II) Using input driver 'libinput' for 'Power Button'
[    13.463] (II) Using input driver 'libinput' for 'AT Translated Set 2 keyboard'
[    13.740]    ABI class: X.Org XInput driver, version 24.1
[    13.740] (II) Using input driver 'synaptics' for 'SynPS/2 Synaptics TouchPad'

$ lspci |grep VGA
01:00.0 VGA compatible controller: Silicon Integrated Systems [SiS] 661/741/760 PCI/AGP or 662/761Gx PCIE VGA Display Adapter

$ xrandr --verbose
xrandr: Failed to get size of gamma for output default
Screen 0: minimum 640 x 480, current 640 x 480, maximum 640 x 480
default connected 640x480+0+0 (0x2ab) normal (normal) 0mm x 0mm
        Identifier: 0x2aa
        Timestamp:  3715368
        Subpixel:   unknown
        Clones:
        CRTC:       0
        CRTCs:      0
        Transform:  1.000000 0.000000 0.000000
                    0.000000 1.000000 0.000000
                    0.000000 0.000000 1.000000
                   filter:
        non-desktop: 0
                supported: 0, 1
  640x480 (0x2ab) 22.426MHz *current
        h: width   640 start    0 end    0 total  640 skew    0 clock  35.04KHz
        v: height  480 start    0 end    0 total  480           clock  73.00Hz

I don't get what's wrong with this.

Looks like the graphic adapter is recognized (it's indeed a SiS 6xx thing), but the screen is not?!

Of course, trying to manually change the resolution doesn't work.

That doesn't make any sense, since during the install, everything was working fine in 1024x768!

Update (better late than never). After several tries, couldn't get better than 640x480 on the long run. I figured out a better resolution could be forced through the xorg.conf, but it didn't last: logging out or bringing the laptop to sleep or (of course) rebooting would simply reset the damn default values. I wish I could post more about this issue (and I wish I could have solved it), but alas the laptop's had to be scraped for parts.

If you encounter this problem with this specific laptop, I'd recommend you search and remove anything that would mess with Xorg configuration as well as any other kind of windows manager.