instead of showing log in screen, external monitor (set as primary) turns black. I can type in the password blindly and it works.

[Tilo Hähnel]

With ubunutu 16.0X it was fine. The error occurred after upgrading to ubuntu 18.04.01

I use a Lenovo Think Pad T460s with a docking station.
Via the display setting configuration I turned off the laptop monitor, using only the two external monitors:

As primary monitor: Fujitsu P27-8 TE Pro (2560x1440) (via display port)
As secondary monitor: Samsung SyncMaster 2433BW (1920+1200) (DVI via Lenovo docking station)

Additionally and may be interesting: I have a crypt setup. When starting the computer, the monitors ask CORRECTLY for the crypt password. Only afterwards, when they should show the log in screen to ask me for my personal log-in password, the monitors turn black (after an extremely short fraction of a second, during which I can glimps a mouse cursor).
When I type in the password nevertheless, the monitor turns on again and I have my desktop and everything.

When the laptop monitor is active, only the laptop monitor shows the log in screen. When turned off, the log in screen does not seem to notice that the monitor is inactive.

My system:

Release 18.04.01 LTS (Bionic Beaver) 64-bit
Kernel Linux 4.15.0-45.generic x86_64
MATE 1.20.1
Processor: Intel Core i5-6200U CPU @ 2.30GHz x4

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