If anyone is still having issues with this, the reason is a bug introduced in the SDK Update that launched with the UK Update. The bug is affected by the names of any items placed in the Scenery Editor if they are too long. The most common way this happens is if you copy something too many times and end up with a name like "OBJECT (copy)(copy)(copy)(copy)(copy)(copy)(copy)(copy)." When you're working in the Scenery Editor after the last update, be very careful not to let the appended copies add up too much. The maximum number you can get away with depends on the length of the name of the object, since the the total length of the name is what causes the crash.
If you need to recover scenery that has this issue, go to the scenery XML for the project and manually rename all the objects with lots of (copy)s. You can completely rename them, or just delete the extra (copy)s. Once you do that your project should open normally and you can continue editing.