Hi Vincent,
here the part of the documentation:
Roles, teams & projects
A Team Member can estimate stories. This works fine.
A Scrum Master has the permissions of the Team Member, plus…
This means for me that also a Scrum Master can estimate stories. This works also.
A Product Owner has the permissions of the Scrum Master, plus…
This means for me that also a Product Owner can estimate stories, because he can do all the things a SM can do.
Or am I wrong?
~Thomas