impossible to run xvfb screen at more than 8bit depth

On an ubuntu 16 server ( SSH )

When launching

Xvfb :99 -screen 1 1024x768x8 &

Works fine, but when trying to launch with 32 bits as follow

Xvfb :99 -screen 1 1024x768x32 &

I am getting that error :

Fatal server error:
(EE) Couldn't add screen 1(EE)

another try with 16 bits, and its another error :

Xvfb: ../../../../include/privates.h:122: dixGetPrivateAddr: Assertion 
`key->initialized' failed.

(EE) Backtrace:
(EE) 0: Xvfb (xorg_backtrace+0x4d) [0x55f95b12f55d]
(EE) 1: Xvfb (0x55f95afb1000+0x1822f9) [0x55f95b1332f9]
(EE) 2: /lib/x86_64-linux-gnu/libpthread.so.0 (0x7f303919d000+0x12890) 
[0x7f30391af890]
(EE) 3: /lib/x86_64-linux-gnu/libc.so.6 (gsignal+0xc7) 
[0x7f3038deae97]
(EE) 4: /lib/x86_64-linux-gnu/libc.so.6 (abort+0x141) [0x7f3038dec801]
(EE) 5: /lib/x86_64-linux-gnu/libc.so.6 (0x7f3038dac000+0x3039a) 
[0x7f3038ddc39a]
(EE) 6: /lib/x86_64-linux-gnu/libc.so.6 (0x7f3038dac000+0x30412) 
[0x7f3038ddc412]
(EE) 7: Xvfb (0x55f95afb1000+0x33538) [0x55f95afe4538]
(EE) 8: Xvfb (0x55f95afb1000+0x1129af) [0x55f95b0c39af]
(EE) 9: Xvfb (0x55f95afb1000+0x7e717) [0x55f95b02f717]
(EE) 10: Xvfb (0x55f95afb1000+0x7dd38) [0x55f95b02ed38]
(EE) 11: Xvfb (0x55f95afb1000+0x7d135) [0x55f95b02e135]
(EE) 12: Xvfb (InitExtensions+0x3d) [0x55f95afe672d]
(EE) 13: Xvfb (0x55f95afb1000+0x12a89f) [0x55f95b0db89f]
(EE) 14: /lib/x86_64-linux-gnu/libc.so.6 (__libc_start_main+0xe7) 
[0x7f3038dcdb97]
(EE) 15: Xvfb (_start+0x2a) [0x55f95afe476a]

Fatal server error:
(EE) Caught signal 6 (Aborted). Server aborting
(EE) 

Where should i look ? Video driver ?

--- EDIT ----

I am now assuming that the resolution or color depth is not supported by the install.

hwinfo --framebuffer command gave :

sudo hwinfo --framebuffer
02: None 00.0: 11001 VESA Framebuffer                           
[Created at bios.459]
Unique ID: rdCR.mCUP8WwEfLD
Hardware Class: framebuffer
Model: "Matrox MGA-G200"
Vendor: "Matrox"
Device: "MGA-G200"
SubVendor: "Matrox Graphics Inc."
SubDevice: 
Revision: "00"
Memory Size: 16 MB
Memory Range: 0x00000000-0x00ffffff (rw)
Mode 0x0301: 640x480 (+640), 8 bits
Mode 0x0310: 640x480 (+1280), 15 bits
Mode 0x0311: 640x480 (+1280), 16 bits
Mode 0x0312: 640x480 (+2560), 24 bits
Mode 0x0300: 640x400 (+640), 8 bits
Mode 0x0303: 800x600 (+800), 8 bits
Mode 0x0313: 800x600 (+1600), 15 bits
Mode 0x0314: 800x600 (+1600), 16 bits
Mode 0x0315: 800x600 (+3200), 24 bits
Mode 0x0305: 1024x768 (+1024), 8 bits
Mode 0x0316: 1024x768 (+2048), 15 bits
Mode 0x0317: 1024x768 (+2048), 16 bits
Mode 0x0318: 1024x768 (+4096), 24 bits
Mode 0x0307: 1280x1024 (+1280), 8 bits
Mode 0x0319: 1280x1024 (+2560), 15 bits
Mode 0x031a: 1280x1024 (+2560), 16 bits
Mode 0x031b: 1280x1024 (+5120), 24 bits
Mode 0x031c: 1600x1200 (+3200), 15 bits
Mode 0x031d: 1600x1200 (+3200), 16 bits
Mode 0x031e: 1600x1200 (+6400), 24 bits
Config Status: cfg=new, avail=yes, need=no, active=unknown

Tried again with several of the resolutions outputed by this command , but still does not work.

Finally had to try several modes and got lucky with

1024x768 (+4096), 24 bits