Edit: Now i have to re-link the group start page to the group. Maybe there is a cron problem ?

Did any other members of the group (other than the group owner) edit these pages since they were added to the group (you can check the History tab for details)?

No, only myself.

Same problem one day later. Now in fact the wiki group start page is missing...

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.

Now it's not possible to set a wiki startpage in the group...

And it's not possible to ptotect the "read only by group" in the protect-tab...

This is working okay for me. The only thing is that the box "Use this as the group's start page" doesn't stay checked. But the page is the start page if you visit the group.php page.

This has been fixed in the current vault/special_tab_edit.php so that forgetting to re-check the box on sub-sequent edits, or group members who don't have permission to see the box, doesn't remove the start page from the group.