Had to search for a long time to find it, because all other comments were unclear.
First, go to Journal -> Lands and check that every area has a progress of 100% (it might not be, even if it shows as fully completed elsewhere).
Then explore the ashen fields again (perhaps multiple times).
After that, the breadcrumb/log people are talking about is actually in the "Unique events" section of the exploration tab. Look for the "Strange Scroll" event, and follow the story from it. You might need to explore many times the same area to get it (and every following step).