Hi Klaus,
I just checked the group rights on your wiki and it seems that there is a error there, nobody has the right to edit pages.
This can be fixed by adding the next line to your localsettings.php in your mediawiki root.
$wgGroupPermissions['*']['edit'] = true;
You could check http://www.mediawiki.org/wiki/Manual:User_rights also, it seems that you're missing the rights for admins, crats, etc etc also.
Best regards, Huib