You may need a properly configured xorg.conf file.

We ran in to this problem with the newer Amiga NG powerPC machines.
Without the proper xorg.conf in place everything froze.
The solution was to install a base Ubuntu-Mate 16.04 with the alternate installer
then copy the xorg.conf then install the Desktop .
A problem developed after with the updated alternate installer not finding hard drives so this became an issue.
The solution was to create an .img files of a base installation with the xorg.conf for each machine in /etc/X11. user just copied the right one to xorg.conf then installed the Desktop.

Later solution was to create a complete .img with the Desktop installed for the various machines and graphics card options. .img can be booted from USB and copied to hard drive .

http://amigaworld.net/modules/newbb/viewtopic.php?mode=viewtopic&topic_id=40682&forum=34&start=80&viewmode=flat&order=0#779592
----------------------------------------------------------------------------------
Section "ServerLayout"
Identifier "Desktop"
Screen 0 "Screen0" 0 0

InputDevice "Mouse0" "CorePointer"
#InputDevice "Keybard0" "CoreKeyboard"
EndSection

Section "Files"
ModulePath "/usr/lib/xorg/modules"
EndSection
Section "Module"
Load "glx"
Load "dri"
Load "dri3"
EndSection

Section "InputDevice"
Identifier "Keyboard0"
Driver "kbd"
EndSection

Section "InputDevice"
Identifier "Mouse0"
Driver "mouse"
Option "Protocol" "auto"
Option "Device" "/dev/input/mice"
Option "ZAxisMapping" "4 5 6 7"
EndSection

Section "Device"
Option "glx" "1"
Option "dri" "1"
Option "dri3" "1"
Identifier "Card0"
Driver "radeon"
EndSection

Section "Screen"
Identifier "Screen0"
Device "Card0"
Subsection "Display"
Viewport 0 0
Depth 1
EndSubSection

Subsection "Display"
Viewport 0 0
Depth 4
EndSubSection

Subsection "Display"
Viewport 0 0
Depth 8
EndSubSection

Subsection "Display"
Viewport 0 0
Depth 16
EndSubSection


Subsection "Display"
Viewport 0 0
Depth 24
EndSubSection

EndSection

[ Edited by Spectre660 29.03.2016 - 15:19 ]