Thanks for supporting, Helen.
Yes, the track of read and unread posts is working fine on the course page, the forum page and also in the forum discussion thread. The problem is in the dashboard only.
The dashboard is really important for us as our students are enrolled in more than 13 different courses and it wouldn't be fair to ask them to visit every course page to check for new posts.
I'll follow your suggestion and check if the 'Forum mailings and maintenance jobs' scheduled task is running ok.