I am having a similar issue:
LoginId:a1d129c4495c7adc1216eba0eefadfe9
EpicAccountId:d051e9df6f8b4ae39ca67bf502ac4293
Assertion failed: (DefaultAlphaTexture != nullptr) && DefaultAlphaTexture->Source.IsValid() [File:D:\build++UE5\Sync\Engine\Source\Editor\LandscapeEditor\Private\LandscapeEditorObject.cpp] [Line: 541]
UnrealEditor_LandscapeEditor
UnrealEditor_LandscapeEditor
UnrealEditor_LandscapeEditor
UnrealEditor_UnrealEd
UnrealEditor_UnrealEd
UnrealEditor_LevelEditor
UnrealEditor_LevelEditor
UnrealEditor_Slate
UnrealEditor_Slate
UnrealEditor_Slate
UnrealEditor_Slate
UnrealEditor_Slate
UnrealEditor_Slate
UnrealEditor_Slate
UnrealEditor_Slate
UnrealEditor_Slate
UnrealEditor_Slate
UnrealEditor_Slate
UnrealEditor_Slate
UnrealEditor_ApplicationCore
UnrealEditor_ApplicationCore
UnrealEditor_ApplicationCore
UnrealEditor_ApplicationCore
user32
user32
UnrealEditor_ApplicationCore
UnrealEditor
UnrealEditor
UnrealEditor
UnrealEditor
UnrealEditor
UnrealEditor
kernel32
ntdll
However in my case, I have used max resolution for my landscape, and the crash occurred after I applied a basic shader to the landscape material that I created and when I switch to landscape mode, it shuts down and shows the above error message.
Actually the editor is crashing even after I reset the landscape material to default checkers, which was not the case before