When i press to (save) in skeletal mesh, i get error:
This skeletal mesh asset has an incompatible Skeleton. Assign a compatible skeleton to this skeletal mesh asset
The solution was to reassign (Skeletal) to the same one 
No matter how stupid it sounds.
When you reassign (Skeletal) via (Assing skeleton), you will probably get an error, you will just need to click ok.
And that’s it, everything should work)
Also came across this error, and a popup asked to also update related animations to proceed. But updating the animations broke them, so just saved the skeleton (backup copy) and restarted the editor without saving the animations.