Hi,
Following this workaround : https://wiki.x2go.org/doku.php/wiki:development:glx-xlib-workaround did the trick !
The workaround also works for ubuntu 18.04. I just replaced "aptitude" by "apt".
I had to chown -R <me>:<me> mesa-18.0.5/ to be able to compile.
Now modifying nextcloud client launch sequence !
Cheers,
Thierry