I have seen that the TICKED "Subscribe forum" means that I am NOT subscribed. Clicking it removes such tick and shows unticked "Unsubscribe forum". Then "User Control Panel - Overview - Manage subscriptions" shows such thread as subscribed. Sorry about such confusion on my side, yet such interface (tick on "Subscribe forum") seems anti-intuitive to me and actually fooled me. It is my suggestion to change the TICKED "Subscribe forum" by an UNTICKED one to mean that subscription is NOT active. That is a suggestion for the admins here.