I now have the same issue. I was importing a Skeletal mesh from Blender with what I thought was the Mannequin rig. In the FBX Import Options window I selected the SK_Mannequin Skeleton. There must have been an issue with my Blender to UE skeleton and it made changes to the SK_Mannequin Skjeleton which is now stuck in T-pose and ruined all my other characters. I had done too many actions to undo back to before I messed everything up.
Can I:
- re-import the original SK-Mannequin?
- reset back to the original SK_Mannequin
- another option without having to rebuilkd a whole project and many characters