Assertion failed: Pair.Value.Num() == NumElements in ...LandscapeGrass.cpp] [Line: 1468]

Hi!
Trying to open a Level or package in a project created in UE 5 Early Access in UE 5.0.3 I always get this error: Assertion failed: Pair.Value.Num() == NumElements [File:D:\build++UE5\Sync\Engine\Source\Runtime\Landscape\Private\LandscapeGrass.cpp] [Line: 1468]

What could I possibly do? I have deleted all grass nodes in materials. What else?

Any help is appreciated. Thanks!