Problem in Subscribing to a Discussion

I need help correcting a problem that has been present for the last couple of weeks. I can no longer subscribe to a Discussion. Upon selecting Subscribe at the top of the page, nothing happens. Further, while viewing the Discussion, selecting the Mark all comments as ‘Read’ button has no effect.

When viewing My Subscribed Discussions, I can Select the Checkbox for a given Title and update my having read the entries by selecting the Mark as read button.

Any help correcting this problem would be appreciated.