Fixed in the current ZIP. This occurs when a user joins or leaves the group - all existing group pages are dissociated. Also, when editing the group, the start page is dissociated. Also, when performing an AJAX section edit on a group page, other-language page, or header page, the data for the edited page becomes dissociated.
To fix these problems, download the ZIP again and replace both vault/special_plugins_group.php and vault/tabs/special_tab_edit.php.
Then, go to the group.php page for the group and edit the settings for that group. Tick the checkbox so that the group can still have group pages. Save, and this should re-associate any messed up group pages.