Well, after experiencing the problem again after reboot and performing some extra
research, I found contributions in the web indicating that this problem is related to
'root' being intentionally prevented (for security reasons) to connect to the
non-root user's X server. The suggested command to overcome this is:
xhost local:root
to which the system responds with
non-network local connections being added to access control list
I tested and it indeed solves the problem, although it is not permanent (you have to run
this command after logout or reboot again to grant root with privileges over the non-root
X server).
Could anyone with knowledge on X servers indicate a robust, permanent solution?
Tks!