I also encountered this problem, some project i can open it normally before but it asks rebuilding when open with UE-5.6.0 , i didn’t change anything to this projects.
the walkaround way i can do is to add a dummy c++ building target for this project (even if this is a pure blueprint project, because visualstudio2022 needs a target to run to invoke the unrealeditor.exe)
This is a sample for GameAnimationSample , i created a empty dummy c++ target named DummyCppTarget for it , and then i can rebuild this project with visualstudio2022 and open again :
PS D:\workprj\uePrj\GameAnimationSample> tree Source /f
Folder PATH listing for volume Workpalce
D:\WORKPRJ\UEPRJ\GAMEANIMATIONSAMPLE\SOURCE
│ DummyCppTarget.cs
│ DummyCppTargetEditor.Target.cs
│
└───DummyCppTarget
DummyCppTarget.Build.cs
DummyCppTarget.cpp
DummyCppTarget.h
PS D:\workprj\uePrj\GameAnimationSample>