Dropbox causing OS X boot hang?

Running early 2011 MacBook Pro (motherboard & HDD replaced on warranty in 2014 due to the video card problem for which there was a general recall and warranty extension. Running El Capitan (10.11.6) and everything fully updated.

The OS X Security Update came through and tried to auto-install per my setup to install updates automatically. Found machine in AM with appearance of having restarted (windows open) and a message that update had failed due to "Dropbox prevented update". I went ahead and manually clicked update on the notice. Machine went through whole update process and hung at about 2/3 of the crawl line (just before the user account log-in page). Multiple retries finally got me to the user account log-in but after entering my password, white sunburst pinwheel spun forever (hung).

Booting in Safe Mode did not change that result. Resetting the SMC and the NVRAM no help. Booting in Recovery Mode, disc checked out fine. So reinstalled OS X from a download. No help!

After discussing my problem with Apple Support I erased my HDD, re-downloaded OS X and then restored from a Time Machine b/u done 1 day before the problem. THAT solved my issue

But of course the OS X Security Update re-presented itself (since my restore predated it and even though I had done that update....with no problem.....to my clean reinstall of OS X before my Time Machine restore.

So this time I turned OFF Dropbox and unchecked "start on boot up" in Dropbox Preferences. Proceeded to do the OS X Security Update and rebooted with NO PROBLEM!! Started Dropbox after boot up and everything ran fine and normal.

Then (with some trepidation!) I turned on "start on boot up" for Dropbox. Sure Enough! Hangs at about 2/3 of the crawl line on attempted restart!! This time I did not try just restarting repeatedly (which had eventually produced the user log-in screen...but not a restart beyond that) but tried just forcing a shut-down and rebooting in Safe Mode.

This time Safe Mode came up (slowly). After it was fully initialized, I started Dropbox and TURNED OFF "start on boot up" (or whatever the exact words are for it to start on system start). I then restarted from Safe Mode, booting normal and NO PROBLEM! I have since rebooted about 4 times and everything seems fine. And Dropbox runs fine once I manually start it AFTER I'm fully booted up.

Seems to me that this is a pretty clear indication that Dropbox, if starting on boot-up, interferes with completion of a full system restart SINCE THE OS X Security Update.

I seem to be running fine now with auto-start turned off on Dropbox, so no immediate problem. But I'd be very interested in any comments, suggestion, etc. Seems like both Apple and Dropbox would be interested to know this about the new Security Update....unless I'm the only guy out there with this problem!