This is the closest existing post I could find that was close to the issues we were experiencing so I figured I would share the symptoms and findings here.
We are preparing up to push Windows 10 (1709) as MS is pushing to release (1803), we are still running SEP client (v 12.12.5.1), which was working for all intensive purposes until recently, for about the past month there have been a handful or so of installs I have completed SOME not ALL which were having problems after applying finishing touches (SEP is the last thing to go on usually prior to the last round of Windows updates).
Symptoms:
Start Menu will not respond
Cortana/Search function in the task bar will not respond
Action Center will not respond
"Will not respond"? = I can left click all day and you don't get anything but with action center, message numbers are still arriving as if everything is normal and you can use a right click on the task bar, bring up task manager, settings etc... you can also use run commands and get to anything needed.
Troubleshooting:
The first Install I was able to get around this, a simple uninstall and re-install seemed to do the trick, no harm no foul and SEP as well as windows seemed to be functioning without issue as well as the other installed applications
That brings me to my next "fresh" install on a brand new computer, this thing has been the bane of my existence the last couple of weeks.
I have tried several installs, re-installs changing install routines etc... all to no avail until last week Thursday when it dawned on me right after I left work, there were only 2 applications installed after the last reboot where everything was working and the final boot up after installing these 2 applications, SEP being one of them.
Monday when I got back into the office, tried removing the other program, reboot, no difference, removed SEP, rebooted, everything was working correctly, ran Clean Wipe, reboot, re-install, again same thing.
A co-worker of mine (one of the admins for our company) has already sent off an email for the latest client version 12.x as this seems to be the practice for this older client version nowadays and we have yet to hear anything from anyone (NOT SURPRISED!) this was yesterday when the email was sent so I get it, takes time, but I'm personally impatient and cant leave something like this alone (especially because I have customers waiting).
So naturally I started digging around to see if anyone else has run into this issue at all recently, came across a few threads where people were having issues similar but not these exact issues nor relating directly to SEP, until I started digging around in the Symantec forums and found this one which seems pretty darn close.
Thanks to those above for giving the push in the right direction...!
Today I found that if I removed the Application and Device Control component everything works as it is supposed (initially it was verified it didn't break anything else as well by removing this, after the fact appeared that the SONAR Heuristic component was failing, but not being reported correctly in the console), once the reboot occurred, there was a "critical Symantec update", the system automatically downloaded, installed and requested a reboot, rebooted, everything was still working.
Now at this point because again I cant leave well enough alone, went in to see if I could re-add to replicate this issue and from my understanding our environment and the way its setup, the app & device control appeared to have been re-installed automatically again, which is good because everything is still working, for good measure, went through the steps of re-adding it through Apps, restarted, Live update ran again, wanted a reboot again, restarted again, again everything is still working properly.
TLDR: Remove the Application and Device Control component, reboot, re-install Application and Device Control component, reboot, live update, reboot, should fix this issue.
Symantec team: Still waiting on that updated Client link...
Shaun