Summary:
In our Moodle 2.3.1 turning Force format on in a collaborative wiki with groups the students cannot edit the wiki or see the pages. In a collaborative wiki with no goups there is no problem with the Force format setting. In collaborative wikis with the group modes separate or visible groups, Force format must not be selected or else the students cannot edit. The tracker issue is MDL-33491.
Rgrds,
Paul.