This issue appeared after upgrading from Ubuntu 16.04 to 18.04.
I have an Intel NUC6i3SYH installed as a headless server on my local network. It has no display and no keyboard attached to it. I control it from my desktop computer, using Remmina. This worked fine when Ubuntu 16.04 was installed on the NUC.
I have now upgraded the NUC to Ubuntu 18.04.01. Suddenly Remmina does not work as intended any more: it can connect to the NUC and asks for the password to get access. But then it only shows a black screen. Remmina only works if I connect a display to the NUC at the time of booting. As long as the NUC senses a display does it provides graphics to Remmina. The graphics in Remmina freeze when removing the display.
What do I need to do to have graphics visible in Remmina without having a display connected to the NUC?
Some technical specs about the NUC: NUC6i3SYH, CPU: i3-6100U (formerly called Skylake), using integrated graphics Intel HD Graphics 520.