Well, there is a bug involved here, but permissions for the topics also come into play. Since 1.3.7sr1, Geeklog checks if the StoryAdmin (or person with similar permissions) actually has the permission to post under the topic s/he chose. This was one of the issues in 1.3.7 - you could create a manipulated version of the Admin story editor and post to a topic that you were not allowed to post to.
So first of all, make sure the topics belong to a group that your StoryAdmin is a member of. Per default, Geeklog assigns new topics to the TopicAdmin group, so change those to belong to the "All Users" group.
Then, there's a bug in admin/story.php in function submitstory(). At line 682, you'll find this:
// Convert array values to numeric permission values
list($perm_owner,$perm_group,$perm_members,$perm_anon) = SEC_getPermissionValues($perm_owner,$perm_group,$perm_members,$perm_anon);
This needs to be moved to the beginning of the function, right after the "globals $_TABLES, ..." line. That should fix it.
bye, Dirk