UE5.1 fails to initialize. Assertion failed: !IsValidChecked(ExistingActor) [File:D:\build\++UE5\Sync\Engine\Source\Runtime\Engine\Private\ActorPartition\ActorPartitionSubsystem.cpp] [Line: 280]

Development World Creation
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I got the same error but only when trying to resimulate. My solution was simular, just selected the instanced foliage actors in the Outliner view and deleted them. Then the resimulation of the procedural foliage worked like a charm.