Forum Subscription Glitch
APC Forums are currently experiencing a glitch where if you subscribe to a subforum, it will no longer be visible either in it's normal location, or via the User CP subscription list. Any such forum is still accessible via posts/threads you have previously created in the subforum. If you unsubscribe, that will restore access via the normal subforum tree.
You can access your previous posts by clicking on your own user name, then "Find More Posts by...". Select a post from the subforum in question, that will get you into that subforum. From there you can unsubscribe if desired.