eZ Webin has an access just to the ezdhtml, not ezoe. The rule can not be modified since the module is not available. Deleting the rule and adding an identical rule for ezoe solves the problem.

Steps to reproduce

1. Fresh install of eZ Webin
2. Add a user to the editor group
3. Login
4. Edit a page with an XML-block
5. Notice that the editor is not available