Resolved Weird bluetooth behavior new iMac 5k late 2015

[hesselMidd]

Bluetooth: not available issues under OS X 10.11.4.

Solution: I tried every possible solution I could find. A NVRAM reset did the job! https://support.apple.com/en-us/HT204063

 

[mroffbeat]

I had the same experience, first when i pre-installed the public beta. Then did a full reinstall of the normal installation. When the update hit, the problem re-appeard.
Resetting NVRAM is the working solution.
The login still is buggy. It looks like a 4K screen inside a 5K screen is showing (double desktop) and disappears after login completes.