We need to take a look at the configuration and errors on why suddenly a few users could not access the Internet. Without looking at logs, it is hard to guess what happen.
Also, do you mean to say: "Why would enabling the Trend Micro http scanning feature disallow internet access after a certain amount of hours?"
Because you mention that you re-enabled the Trend Micro today, so I assume that your question meant to say as above.
Those users who can't access the Internet, how many users are they, and also I assume there are some other users who can access the internet with no problem at all?
What did the logs say when some users are not able to access the Internet?
And what have you modified to allow access back?