Strange sometimes black and sometimes bright spots in the horizon, help

Hi there.
In the last few weeks, I have had this problem with my world partition level. Apparently randomly, these black spots appear on the horizon and start increasing in size until they take up the whole screen or until I move the camera away from them.

Sometimes, instead of black, it’s this blinding whiteness.

I can’t seem to find a solution.
Any help or suggestion is much appreciated, thanks!