Problem with the output to the external Monitor

Hello Trisquel users !

I am facing an "issue" after upgrade from toutatis to belenos (the upgrade went smoothly, thanks quidam for making smooth transition !), which results in my external Samsung VGA LCD monitor not getting any output from my laptop's VGA port. Previously I had a virtual dual-screen setup, which worked, and the external monitor got the picture, when I started X/gdm (not before) but now the command "xrandr" which is supposed to list the outgoing outputs gives an error:

root@trisquel# xrandr
xrandr: Failed to get size of gamma for output default
Screen 0: minimum 1366 x 768, current 1366 x 768, maximum 1368 x 768
default connected primary 1366x768+0+0 0mm x 0mm
1366x768 0.0*
1368x768 0.0

Previously it listed "VGA-0" as secondary output which went to my external monitor, so I was able to use the virtual screen solution there. I should add that the /etc/X11/xorg.conf file hasn't changed during the upgrade and remained the same as being used in toutatis.

root@trisquel# lspci -v |grep VGA
00:01.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Wrestler [Radeon HD 7340] (prog-if 00 [VGA controller])

I already googled up the "xrandr" error, but couldn't find anything related that would fix my issue, I assume that it could be maybe a (proprietary) driver related issue ?

Any hints are appreciated, I can paste more commands if needed to investigate the error ...

Thank you !