Adding tasks to SO's for restricted users

[Guido1982]

This may be some simple thing I overlook, but I can't find it. I want to give a certain profile/group access to create a task from the related modules below a Sales Order. So I enabled 'create/change' on tasks for this group. This works for almost every module now (like accounts, helpdesk), but not for Sales Orders. Is there a special setting I need to set to make this work? I can do this from my account (admin), so it must be possible.