The ghost concrete tiles appear even if my network has no concrete and there are more orders than robots and if i add more robots then the amount of orders will increase, too.

Yes, the current implementation has some overshoot based on available concrete and available robots (specifically for large numbers of roboports). This should be less of a problem for mature bases where some roboports have fully filled their area. It's tricky to find a good balance between concrete laying speed, overshoot, and game performance.

My factory is running with many construction robots (Last I checked is 10000) with it covering vast amounts of lands. Any chance you could implement some sort of sanity checker as to not act when there's more than say,,, 2000 robots already working?

I looked at your code:
the checkRoboports() goes through the list with the roboports.
after the validaty checks it will order 10% of the available robots to place the concrete. But this will lead to an overload:
20 Roboports(= 20 Iterations), 10 Available Robots:
In the iteration it will at least order 1 robot.
1st Iteration 1 robot, 2nd iteration 1 robot,...
It will order 20 Robots, but only 10 are available

I dont know but i exspect that the roboports logistic network id should be accesable. So you could identify all diffrent Networks put their id with the respective available roboports to a list.
Then you could count the robots you order to place tiles, when available robot count it reached no more orders are added to that network.

Fixed in 0.9.4