Can't launch graphical apps from terminal after updating to 15.10

As a regular user, run the command xhost +. After that, try running your graphical application again.

This turns off host-based access control on the X server. Here is the relevant information from the man page:

The xhost program is used to add and delete host names or user names to the list allowed to make connections to the X server.

+ Access is granted to everyone, even if they aren't on the list (i.e., access control is turned off).

Odd but export DISPLAY=:1 helped. Cheers.

Warning: Improper use of xhost can inadvertently give every host on the Internet full access to an X display server.

Generally you shouldn't turn off host-based access control on the X server unless you know what you're doing. Instead you should only allow the things that require access.

Solution:

Execute: xhost +si:localhost:root

The above solution also works for Wayland users executing an elevated XWayland application.

When attempting to run an XWayland application with elevated permissions you may receive the following error:

No protocol specified
Unable to init server: Could not connect: Connection refused
Could not parse arguments: Cannot open display:

In my case, the same problem found after updating a single package gnome-shell through synaptic-package-manager. Immediately after next login I was not able to access any GUI part of the OS. Did lots of searching, finaly,

when I used apt install gdm3 from recovery mode terminal, I was able to find a login prompt after system boot(Only terminal, not GUI).

After that I tried apt install gnome which installed a whole bunch of components along, once downloads finished, I was able to get my OS in GUI mode.

If you're not getting login prompt itself then try to follow above step in recovery mode. Otherwise try installing Gnome which updates the gdm3 along.