So only with CASM not if you save normally? I think it would be good to check over the CASM settings to make sure it doesn't have anything set on or off that could potentially corrupt your save. But the most likely thing thats corrupting it is the mods you have so try just removing some mods and see if you can save then go through those mods and find which one was causing the corrupted saves. or just trun them all off and go through them and see which one is doing it.