Yes... you shouldn't have any issue.
I wonder, then, if UserActivity session is getting truly set.
On the page that it is supposed to work on, can you echo out the session.UserActivity above the HTML and see what comes up. My guess is that it is not getting set properly.
I ran into a similar issue when I created an "active" column in one of my user DBs... since we know the User Level appears to work, I have a feeling this a problem with the other session variable.
After you echo the other session, let me know your findings. (you may need to remove the access rules for the time being).