recently i exported a terrain from an older project as a unity package and re-imported into a project running the latest and greatest 2018.3. this worked well on my local machine but noticed other machines pulling the changes would throw ‘Unknown error occurred while loading terrain asset’ repeatedly in the console, with the terrain then obviously not displaying.
the only “fix” i’ve found for this is to import the package on each subsequent machine. this doesn’t really solve the issue though as if we want to touch up areas / re-paint anything and commit those changes, the issue persists. so we’re basically just left with recreating the terrain from scratch in 2018.3, unless someone else here has other suggestions 
i will also say that the terrain.asset file shows modifications in git, whether or not the asset was modified, and there seems to be no way to reset your branch / discard or otherwise stash the changes in this situation; as far as i can tell, there should be no process running that would be preventing this…