When I play Fallout 4, Windows always bugs me with a dialogue to reduce the screen effects in order to increase the performance.
After some fruitless research I gave up, so that when I want to play something I just have to deal with the internal stupidity of Windows.
The last time, Fallout 4 was not able to recover after being minimized an refocused with the wrong resolution on my secondary monitor. Not so nice.
Since this now happened the second time, it's time to archive the way I can fix this issue:
Fallout 4 launching on the wrong display.
Google is the answer, and a short query immediately points the to the following page, where there are some solution strategies.
Edit the File Fallout4Custom.ini in the directory "C:\Users\%USERNAME%\Documents\My Games\Fallout4"
Search for the following values and edit them to your needs
Since i need the game to be on my primary display, iLocation X = 0 and iLocation Y = 0 suits my needs. Otherwise just add or subtract the screen size depending on your screen resolution and screen adjustment.