Assetion failed when opening a 5.0.3 project in Unreal Engine 5.1 after cleaning and recompiling

Greetings !
I am stuck using 5.0.3 version of Unreal engine because of a crash when opening my project in 5.1 version.
Error Message is :
Assertion failed: Model [File:D:\build++UE5\Sync\Engine\Source\Runtime\Engine\Private\Animation\AnimSequenceHelpers.cpp] [Line: 192]

I have try eveything I found :
-Empty AppData directories for UE 5.1
-Setting VS project by switching to UE 5.1 and generate visual studio files and regenrating project with succes build
Anything I do, always same error !

I have lost days and it is hell to endure the crashy version 5.03 any more !
Please , help greatly needed !

Here’s some more details :

At line 192 in source file Private\Animation\AnimSequenceHelpers.cpp , there is only this :

check(Model);

So is there somebody here to find out what does it means ? How to have more details about what is wrong ?

Assertion failed: Model [File:D:\build++UE5\Sync\Engine\Source\Runtime\Engine\Private\Animation\AnimSequenceHelpers.cpp] [Line: 192]

UnrealEditor_Engine
UnrealEditor_MeshBoneReduction
UnrealEditor_MeshBoneReduction
UnrealEditor_MeshUtilities
UnrealEditor_MeshBuilder
UnrealEditor_MeshBuilder
UnrealEditor_Engine
UnrealEditor_Engine
UnrealEditor_Engine
UnrealEditor_Engine
UnrealEditor_Engine
UnrealEditor_Engine
UnrealEditor_Engine
UnrealEditor_Core
UnrealEditor_Core
UnrealEditor_Core
UnrealEditor_Core
UnrealEditor_Core
UnrealEditor_Core
UnrealEditor_Core
UnrealEditor_Core
UnrealEditor_Core
UnrealEditor_Core
UnrealEditor_Core
UnrealEditor_Core
kernel32
ntdll