I’m late to the party, but I had exactly the same problem.
Had a project not opened for a few weeks, and have the same error in the 4.24.3 project.
Tried to fix the asset somehow because I don’t have a backup, and because it is the only version ever checked into our source control. So the files itself is not corrupted.
The only way I could “save” the asset was to create a new project in 4.25.4 and copy the uasset file into the project folder. It loaded fine there. Then you can export the fbx (or whatever you asset is) and reimport it back to 4.24 hopefully.
I think there was a bug introduced in 4.24.3 maybe that causes the SkeletalMeshReduction error on load? Whatever it is, it’s fixed in 4.25 apparently.