I am not sure why you would be receiving the security token issue. There is a possibility of this happening if you used the browse backward in history of your browser to a post you were in process of posting or editing, or a similar scenario. Other than this, it might be related to your browser's cookie settings for this site.

If you wish to find a topic you are subscribed to, you can click on UserCP top-left of screen to find your User Control Panel. In this window find the Subscribed Threads heading in the column on the left side and click on List Subscriptions. This will list threads you have subscriptions to. By default members are subscribed to threads they created or posted to. Threads can also be manually subscribed to via Thread Tools.