This is a suspected bug in the forum notifications rather than the CMS software.
I subscribed to someone else's post and was notified of an update.
The "Click here to view forum on-line" link in the e-mail message was simply:
<a href="?ForumID=4&Thread=00003835">Click here to view forum on-line</a>
There is not a full path in the href so it does not work.
I'd love this to be fixed if it is easy because I'd like to track a few threads and easily link back to them.