Problem with deleting a reply in group discussion


I am using Elgg 2.1.1

In a group, when I try to delete a reply on a discussion (using discussion ang grouo tools plugin), I have this error:

"L'action demandée (discussion_reply/delete) n'est pas définie dans le système."

Impossible to delete, even when connected as administrator.

Do you have an idea?

Thanks for hepl