Repro steps:

1. Open Unity and create a project
2. Go to "Edit" -> "Preferences..." -> "Cache Server" -> Change "Cache Server Mode" to "Local"
3. Go to "File" -> "Build Settings..." -> "Player Settings..." -> Change the "Scripting Runtime Version" to "Experimental (.NET 4.6 Equivalent)"
4. Restart Unity
5. Create a Cube in the Hierarchy window
6. Save the scene
7. Exit Unity

Expected result: Unity should close when you press the "Close" button
Actual result: Unity freezes and needs to be closed through the "Task Manager"

Reproduced with: 2017.1.0a6, 2017.1.0b7, 2017.2.0a1
Did not reproduce on: 5.6.1p1(As this doesn't have the new scripting runtime)

Note:
The bug happens when anything is created/deleted/modified, due to it being uploaded to the cache server.
IMPORTANT!
As the local cache server has bugs (#911116) that prevent it from working properly, you have to change where it is saved. For convenience, save it directly to any disk in a folder called "Cache"