To reproduce:
1. Open user's project
2. Go to 'Unity'->'Preferences'
3. Go to 'Cache server' tab and set mode to 'Local' (Do not click anywhere else, just once, when choosing Local mode)
4. Click on editor window and close Unity
5. Repeat 1-3 steps, you can see, that Cache server mode is disabled.
6. Set cache server mode to 'Local', click on 'Unity preferences' window anywhere, close Unity.
7. Repeat 1-3 steps, you can see, that Cache server mode is Local now.

Expected: it should be set to 'Local' both times
Actual it is set to 'Local' only the second time
Note: If you do these steps on changing external script editor, it will change immediately

Reproduced on: 5.5.0a6, p1, p2, p3, 5.6.0a1, b1, b3
Not able to reproduce on: 5.4, 5.3 (there is no drop-down menu) for cache server

Tested on MacOS Sierra 10.12