I have seen various answers to similar questions for previous versions of Mac Os X 10.11, but apparently nothing that is relevant to my problem or that has been useful so far.
I have a Macbook Pro, late 2012. Multiple attempts to update to the latest Mac Os (10.11.5) from the App Store have failed with “verification failed” when starting the actual installation after download. I’m quoting below a part of the install.log file that seems to be relevant but from which I cannot understand what is going on. Please note that the very same installation procedure worked on my newest Macbook Pro, mid 2015; the failing machine is the previous one I used, which I’m now using from home. (Perhaps, a detail that may be relevant, from what I read from previous users with similar problems, is that I upgraded my McP 2015 from my office’s network while I’m using my home ADSL for the failing McP 2012).
If you also encounter this problem, I suggest you may benefit from downloading the COMBO (combination) updater and using that:
Download OS X El Capitan 10.11.5 Combo Update
(Sorry, but it’s a very big download – 1.5 GB)