Glitch outside the map (Zoo Tycoon 2)

I got an annoying glitch again, or maybe not a glitch ...
When you open a map (any), then outside is not a city, not a beach - NOTHING ... there is just either a black background or a blue one (sort of).
This is unnerving because when you walk, it's like fog, and when you go to the limits of the map, you get goosebumps...
What should I do? How to remove?