Worked it out.
In the admin panel:
then tick the 'upload files' box for registered users.
NodeBB version: : 1. 11.1
NodeBB git hash:
Database type: mongo
Database version: v4.0.1
Exact steps to cause this issue:
There is a very popular topics, users often ask me why he has been reminded.
Can I see the users who are interested in this topics?
Ah, not really... topic subscriptions are considered (in my opinion) user private data, and that is not shared by default.
You could instruct your users to adjust their settings if they are being notified about a specific topic too often: