How can I unlock the "Trials of War"?

I am way late to the party, I know, but I started playing Shadow of Mordor only a few weeks ago.

I have completed both the main game and the first 2 DLCs. I now wanted to do the "Trials of War", but every time I try to start one I am greeted by this message:

Locked: Play the Story Mode Introduction first

The thing is that I have already played that, and one savegame is the fully completed storymode.

How can I unlock this content?

If it makes a difference, I have bought the GOTY edition on Steam.