This is caused by adding graphics (DRM) devices dynamically after the device is connected, and it is a regression which seems to be solved again in Ubuntu 18.04 where such behaviour is no longer observed.
There is a workaround that you can use if you see this problem. It effectively causes more DRM devices to be present in the system even before you connect a DisplayLink device. This is similar to having a traditional graphics card present in the slot, but no monitors connected to it.
Use evdi module parameter
In DisplayLink Driver for Ubuntu version 4.2.x, there is a new module parameter which can be utilized to increase the number of DRM devices at module load time, without delaying this action to the moment where the Software reports DisplayLink device connected.
Steps to take:
/etc/modules to always load it when the system starts:
# echo evdi >> /etc/modules
Indicate how many DRM devices should be created initially by evdi. This number should generally be equal to the number of screens you are connecting to DisplayLink devices. For example, for two extra screens use the following:
# echo "options evdi initial_device_count=2" > /etc/modprobe.d/evdi.conf
After rebooting, even when no DisplayLink devices are connected, you should see two additional
/dev/dri/card* entries. DisplayLink Software will use those existing devices, rather than creating new ones.