-e:

-repro:
1) create new project
2) switch to FB platform
3) close editor
4) delete ivy.xml under <project folder>\Library\FacebookSDK
5) start editor
6) get this error:

Falling back to default Facebook SDK. Failed to import FacebookSDK from 'Library\FacebookSDK' with error: System.IO.FileNotFoundException: Could not find file "D:\Work\Bugs\FB_corruption2\Library\FacebookSDK\ivy.xml".

7) open player settings
8) try switching FB sdk
9) get this error and a stuck progress bar:

UnauthorizedAccessException: Access to the path "Library\FacebookSDK\Facebook.Unity.Settings.dll" is denied.