External Monitor not working. Ubuntu, nvidia

I'm having a precision 7530 with i7-8850H and NVIDIA Quadro P2000 and using ubuntu 18.04. Now after some time the external montior connected over displayport (displayport to hdmi cable, where the displayport goes into the laptop and the hdmi cable into the monitor) stopped working. I'm using this workstation at work with the TB18DC dock, where 2 external montiors are connected and work properly. But at home I do not have any dock and with the setup mentioned above the external monitor stopped working. It did work some months ago. During the time it worked and now I have done normal ubuntu software updates where also the bios-firmware has been updated to 1.5.2 (i'm not sure what version i've had before, was the first time i updated firmware after bought, so maybe 1.0.7?).

I have also installed the nvidia 390 driver.

xrandr says with connected displayport monitor:

Screen 0: minimum 320 x 200, current 1920 x 1080, maximum 8192 x 8192
eDP-1 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 344mm x 193mm
   1920x1080     60.03*+  60.01    59.97    59.96    59.93    48.02  
   1680x1050     59.95    59.88  
   1600x1024     60.17  
   1400x1050     59.98  
   1600x900      59.99    59.94    59.95    59.82  
   1280x1024     60.02  
   1440x900      59.89  
   1400x900      59.96    59.88  
   1280x960      60.00  
   1440x810      60.00    59.97  
   1368x768      59.88    59.85  
   1360x768      59.80    59.96  
   1280x800      59.99    59.97    59.81    59.91  
   1152x864      60.00  
   1280x720      60.00    59.99    59.86    59.74  
   1024x768      60.04    60.00  
   960x720       60.00  
   928x696       60.05  
   896x672       60.01  
   1024x576      59.95    59.96    59.90    59.82  
   960x600       59.93    60.00  
   960x540       59.96    59.99    59.63    59.82  
   800x600       60.00    60.32    56.25  
   840x525       60.01    59.88  
   864x486       59.92    59.57  
   800x512       60.17  
   700x525       59.98  
   800x450       59.95    59.82  
   640x512       60.02  
   720x450       59.89  
   700x450       59.96    59.88  
   640x480       60.00    59.94  
   720x405       59.51    58.99  
   684x384       59.88    59.85  
   680x384       59.80    59.96  
   640x400       59.88    59.98  
   576x432       60.06  
   640x360       59.86    59.83    59.84    59.32  
   512x384       60.00  
   512x288       60.00    59.92  
   480x270       59.63    59.82  
   400x300       60.32    56.34  
   432x243       59.92    59.57  
   320x240       60.05  
   360x202       59.51    59.13  
   320x180       59.84    59.32  
DP-1 disconnected (normal left inverted right x axis y axis)
DP-2 disconnected (normal left inverted right x axis y axis)
HDMI-1 disconnected (normal left inverted right x axis y axis)
DP-3 disconnected (normal left inverted right x axis y axis) 

I have also tried the following:

• installing lightdm and use it (no effect)
• disable switchable graphics in bios (no effect)
• downgrade firmware version-by-version till 1.2.5 (as it won't let me downgrade back to 1.0.7. no effect)
• reset bios settings to default (no effect)
• reinstalling nvidia-driver (no effect)
• trying without any nvidia-driver (no effect)

I booted from a live ubuntu usb stick and without external montior it worked normally but with external monitor I got the following error message:

Any ideas how to get the monitor working? Thank!