Hello,
It is very important in your case, to check if this issue is occurying due to SEP or not.
Question arises, why did this happen only after upgrading to version 12.1 and why was this working with SEP 11??
1) Could you let us know if there were anything else which were changed on your machine other than upgrade to SEP?
2) How many machines is this issue occuring on ?
3) What Software are you using?
4) In what way is it using IE?
5) Could you uninstall Network Threat Protection with Application and Device Control from the machine and check if that works?
6) Symantec does not use IE other than Downloading definitions updates and so on.
7) As Martin suggested, was there any ADC policy set up?
8) If you could try uninstalling IE and reinstalling IE and check if your software works??
Hope that helps!!