Users can subscribe only to questions they created.
Steps to reproduce:
1.-Create a role that allows to subscribe to message boards messages and categories
2.- Assign that role to user1.
3.- With user2, create a question
4.- With user1, view that question
user1 can subscribe to changes in that question
No button for subscribing is appearing
After some investigation, it looks like the API is not exposing the available action even though by permissions, the user can perform it.