The sorrounding lots go back to default

Since a few weeks, I have the same problem. Whatever save I trie to play, after some time spent on playing, all the neighbourhood lots are shown as their default version. The lot I play in works most of the time, but sometimes it is also empty when I load it. But the whole neighbourhood ist on default, which is extremly annoying, because I spent hours on making it look good. Does anyone else have this problem? I already reinstalled my game, tried different saves and restarted my game a lot of times!