Okey, I found the reason..... some time ago, I tested Mod Gui.... and disabled eye candy features.  It seems this option is still active when mod is disabled. I need to activate autociv and Mod-Gui [0.28.18] to have the option back in the menu, and with enabling eye candy features the waterfalls are back again. ( Well I have not seen this setting left back in user.cfg)  I did not have this in mind because I thought with disabling these mods I can eliminate this reason.

@Sundiata trying to inbox you about an artwork of Amanirenas I'm looking for but my account may be too new

This really points to the work network or router rather than the game itself. UDP traffic being blocked would explain why some players can connect while others cannot, especially since it only happened at work.

It runs fully sequentially, but there is good potential to parallelize parts of it. This is something I'm working on actually.   Yes, I think so. Sadly, I personally don't know enough about it to implement it myself, but I know that there have been experiments/attempts by others in the past.

Depends on what. Rendering and simulation are on the same thread, pathfinding runs in multiple threads.  Sound is in its own thread. The OS decide which threads goes onto which core. @vladislavbelov is working on a way to run the rendering on parallel to the next simulation turn. That will help although on higher end machines, rendering isn't so hard on the GPU.