16.04 - Dual-Monitor Setup - only one Monitor is working - Ask Ubuntu

i installed second de yesterday (xfce4) , since then, notebook/ubuntu 16.04 lts recognizes 1 screen @ time.

if plug in vga kabel, screen working external monitor , vice versa if boot unplugged vga kabel, laptop screen works. plugging in/out while running os of vga kabel doesn't make difference. furthermore, feels way slower , laggy plus unity stopped visual treats transparency in sidebar etc.

measurements taken already:

• removing xfce4
• reinstalling drivers (nvidia, intel)
• reinstalling lightdm
• reinstalling unity
• reinstalling xorg

log information:

only suspicious things able detect in xorg.0.log

    [     6.073] (==) no screen section available. using defaults. [     6.073] (**) |-->screen "default screen section" (0) [     6.073] (**) |   |-->monitor "<default monitor>" [     6.074] (==) no device specified screen "default screen section".     using first device section listed. [     6.074] (**) |   |-->device "card0" [     6.074] (==) no monitor specified screen "default screen section".     using default monitor configuration. [     6.074] (==) automatically adding devices [     6.074] (==) automatically enabling devices [     6.074] (==) automatically adding gpu devices 

xrandr query:

xrandr --query xrandr: failed size of gamma output default screen 0: minimum 1920 x 1080, current 1920 x 1080, maximum 1920 x 1080 default connected primary 1920x1080+0+0 0mm x 0mm    1920x1080     77.00*  

oddly enough, seems it's running on refresh rate of 77.00 hz , when reboot without vga connection, displays error display configuration.

i can not determine issue right now. searching similar problems , bugfixes didn't far. have idea going on?

with best regards,

23ro