Are you trying to unsubscribe via the "thread tools" drop-down menu from the top of that thread or from your User CP's "Subscribed Threads" option?
I'm wondering if perhaps you were subscribed to a thread that was combined with another one, so that the thread ID number you'd subscribed to may not exist. Whichever method you were attempting to unsubscribe with, please try the other and let me know if it works.
To unsubscribe via Subscribed Threads in your User CP, cick "list subscriptions" then click the box to the right of the thread you want to unsubscribe from, then select "unsubscribe" from the drop-down menu at the bottom of the list of threads.
If you have any questions, feel free to PM me or post to this thread.
I always unsubscribe from the latest post received in my Inbox, but it didn't work and I was getting that error message the other day. Just unsubscribed in the same way from another thread and it worked.