Okay I was able to find the root cause by learning more about the different debugging overlaysand fix it.
With the waterfill mod we surrounded our base with a ring of water. That mean biters can't come close and attack us.
Unfortunately we left one single tile open by accident. The biters ...

Okay, here is an update:
I have spent some time better understanding the debug overlay and what I found is that shortly after the waterfill/landfill is placed,
the Pathfinder is going crazy and it seems that this is happening on server and on client side.
Not sure if it is important but I already ...

I have a savefile (with mods) with around 200 hours (150MB currently) on my linux server and recently (without any changes or new major builds) things started to get crazy.
The thing I want to report here is that every time we did not use landfill (or waterfill mod) for a few minutes and then using ...