Unfortunately the latest Nvidia driver (340.107) for GeForce 210 that I installed on my Ubuntu 19.04 x64 system is unstable. Every time when I turn off the HDMI screen that I have connected to it, or when I switch to a different input, the system freezes and I only see a white underscore in the topleft corner. After the freeze the system is not reachable via SSH and is not pingable.
Found the culprit. Thanks for your suggestion Generix, I tried it but it does not make a difference for the following obvious reason. Apperently it does not freeze on the moment of switching HDMI input, but it freezes on the moment that the Harmony remote sends a infrared command that sets my Ubuntu in a shutdown state (although it does not turn of nicely).