Zsolt,
Go to %appdata% and navigate to \Microsoft Flight Simulator\SceneryIndexes\
Delete everything in there, the scenery indexes will be recreated by the sim after restarting it.
Also, I don't know what the specs of your rig are, but you might want to take the scenery settings down a notch. Turn them up too high for your rig and anomalies will occur.
Cheers,
Jan
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