Q: "OS X El Capitan Update 10.11.6" doesn't install

My App Store shows the following update:

But when I click to update it opens the notification dialog "Restarting Your Computer" with the 60s countdown:

If I don't click anything and let the counter go to 0, it automatically closes the dialog and shows another dialog within the App Store, "Available updates have changed".

Finally, nothing happens if I click any of those buttons.

Then I followed the instructions given by Linc Davis in Re: app store download update but it doesnt install sometimes, but it doesn't seem to change the behaviour I'm seeing - although it really seems to erase App Store's cache.

After I delete /var/folders/zz/zyxvpxvq6csfxvn_n00000s0000068 and reopen App Store, this is what I see in syslog (long log with lots of repeated messages). And the following is what I see in syslog after I click Update and let the countdown go to 0:

Sep  2 10:47:15 jardel-mac softwareupdated[548]: libcoreservices: _dirhelper: 400: mkdir: path=/var/folders/zz/zyxvpxvq6csfxvn_n00000s0000068/C/ modes[2]=0700: Operation not permitted

Sep  2 10:47:15 --- last message repeated 2 times ---

Sep  2 10:47:15 jardel-mac sandboxd[140] ([548]): softwareupdated(548) deny file-write-create /private/var/folders/zz/zyxvpxvq6csfxvn_n00000s0000068/C

Sep  2 10:47:15 jardel-mac sandboxd[140] ([548]): softwareupdated(548) deny file-write-create /Library/Caches/com.apple.SoftwareUpdate

Sep  2 10:47:15 jardel-mac sandboxd[140] ([548]): softwareupdated(548) deny file-write-create /private/var/folders/zz/zyxvpxvq6csfxvn_n00000s0000068/C

Sep  2 10:47:15 jardel-mac sandboxd[140] ([548]): softwareupdated(548) deny file-write-create /Library/Caches/com.apple.SoftwareUpdate

Sep  2 10:47:15 jardel-mac sandboxd[140] ([548]): softwareupdated(548) deny file-write-create /private/var/folders/zz/zyxvpxvq6csfxvn_n00000s0000068/C

Sep  2 10:47:15 jardel-mac sandboxd[140] ([548]): softwareupdated(548) deny file-write-create /Library/Caches/com.apple.SoftwareUpdate

Sep  2 10:48:11 jardel-mac WindowServer[305]: send_datagram_available_ping: pid 548 failed to act on a ping it dequeued before timing out.

Do you have any suggestions or directions?