Seems that the builds even the fast ring releases need to be tested more. I have been installing Fast Ring builds for years and the newly released build 14986 is the first one which refuses to install properly.
As I write this there is a 24 page thread with various suggestions from end users including registry hacks to disable Unified Update Platform (UUP) which is apparently in this build. Some have had success while other like myself simply see this build go through the process of installing the update only to have it revert back out. No error messages are displayed.
So far except for the initial post by a Microsoft Forum Moderator/ Support Engineer no followups over the last 3 days from anyone from Microsoft. They have asked for specific information regarding the existing boxes and configuration but that is as far as it has gotten.
I just hope that if this installation issue is related to UUP which appears at least in some cases it is, since registry hacks that bypass UUP, to allow the upgrade to work does not become another AUP.
I have seen this same thing happen with AUP on some systems where the upgrade fails, reverts back to the prior build and without any indication to the end user what the issue is. The system then tries to install the same update over and over again.