Crash with editor when trying to paint foliage. All foliage was assigned to a DataLayer (Editor), and is spatially loaded enabled on all landscape instances. The scene also included Procedural Foliage Spawner.
Assertion failed: !IsValidChecked(ExistingActor) [File:D:\build++UE5\Sync\Engine\Source\Runtime\Engine\Private\ActorPartition\ActorPartitionSubsystem.cpp] [Line: 280]
Tried without success:
Make DataLayer visible
Make all Landscape instances visible
Disabled Is Spatially Loaded on all Landscape instances.
Removing all ProceduralFoliageSpawner
The Solution for me was to delete the DataLayer containging the foliage.