Aggiorno questo post nella speranza di trovare una soluzione, con sorpresa scopro che la funzione glxheads funziona perfettamente pur restituendo questo log:

pippo@ubuntu:~$ glxheads
glxheads: exercise multiple GLX connections (any key = exit)
Usage:
glxheads xdisplayname ...
Example:
glxheads :0 mars:0 venus:1
libGL warning: 3D driver claims to not support visual 0x24
libGL warning: 3D driver claims to not support visual 0x28
libGL warning: 3D driver claims to not support visual 0x2c
libGL warning: 3D driver claims to not support visual 0x30
Name: :0
Display: 0x10027008
Window: 0x2a00002
Context: 0x1002faf8
GL_VERSION: 1.2 Mesa 6.3.2
GL_VENDOR: VA Linux Systems, Inc.
GL_RENDERER: Mesa DRI Rage 128 20041026
pippo@ubuntu:~$

mentre sia glxgears che glxdemo restituiscono lo stesso errore:

pippo@ubuntu:~$ glxgears
libGL warning: 3D driver claims to not support visual 0x24
libGL warning: 3D driver claims to not support visual 0x28
libGL warning: 3D driver claims to not support visual 0x2c
libGL warning: 3D driver claims to not support visual 0x30
pippo@ubuntu:~$

pippo@ubuntu:~$ glxdemo
libGL warning: 3D driver claims to not support visual 0x24
libGL warning: 3D driver claims to not support visual 0x28
libGL warning: 3D driver claims to not support visual 0x2c
libGL warning: 3D driver claims to not support visual 0x30
Resize event
Resize event
Redraw event
Redraw event
pippo@ubuntu:~$


Questo potrebbe far pensare che sia un problema legato esclusivamente al 2D ?