i just wanted to post it here so that if someone encounters a similar problem s/he can benefit from my solution:

My setup:
Windows 10
Dual Monitors
- 1920 x 1200 (secondary)
- 1920 x 1080 (primary)

Problem:
Starting the game leads to a black screen and after a few seconds the game minimizes itself.
Attempts to focus the window again (click on the game in the taskbar) only lead to black screen and secondary monitor flickering for a few seconds and then minimizing the game again.

Solution A:
- Disable the primary screen in windows-settings
- edit the config file ("C:\Users\<yourusername>\AppData\Local\HandyGames\com.hg.townsmen.rebuilt\savegames\game_config") and set the "correct" resolution for the secondary (now the only) monitor. Just deleting the file did not help me somehow.
- start the game
- change the display settings to windowed
- maximize the window

Solution B:
- edit the config file ("C:\Users\<yourusername>\AppData\Local\HandyGames\com.hg.townsmen.rebuilt\savegames\game_config") and add the line `windowMode = 3`

Bug still persisting:
this is just a workaround, the game STILL does not work neither in fullscreen nor in borderless window mode on my primary window.

Thank you for your post.
It helped me figure out a solution for my problem (Townsmen Rebuilt v.2.2.6):
Townsmen would always start as maximized window with a 4k resolution, independent of which settings I would apply.
The in game Options menu only offers: Fullscreen, Resizeable window, Borderless Window. Neither of them accepts a fixed size. Resizeable Window does not remember the size it was set to when the game was shut down. The option 'Windowed' is not listed.
The usual key combinations like ALT+ENTER do not work to force it into window mode.

However, it is possible to do it via configuration:
Edit the config file ("C:\Users\<yourusername>\AppData\Local\HandyGames\com.hg.townsmen.rebuilt\savegames\game_config")

Set the entries:
resolutionHeight = 720
resolutionWidth = 1280
windowMode = 0

For width and high you can pick your destination resolution.
Window Mode 0 forces it to the fixed size window mode.

If you restart the game and go to the Options menu, it will now state Window mode: Windowed. (not resizeable Window)
If you click on the entry Resoultion, you will get a drop-down menu with a list of supported Resolutions to choose from.
These settings will be saved once you shut down the game.