Creating landscape splines is extremely unstable, sometimes UE4 will crash or hang, it has also caused the map to crash on load. I’ve done this in 4.7 and it worked.
Can you post a copy of your crash log from one of these crashes? They can be found at \Unreal Projects\PROJECTNAME\saved\logs. Additionally, does this occur in a clean, blank project with no additional content or is it limited to one project?
We have not heard from you in several days. I am marking this thread as answered for tracking purposes. If you experience this error again, please comment with the requested information.