It is very possible that a plugin is interfering with this.
Add the following code to your includes/config.php file to determine if this is the case,
Then check to see whether or not your moderator is now able to access the ModCP.
Be sure you add the code directly under <?php
Code:
define('DISABLE_HOOKS', true);
If your moderator is then able to view the ModCP correctly, there's an issue with on of your plugins. If your moderator is still not able to view the ModCP correctly, we can further look into this issue.
Beware that the above code temporarily disables all plugins. Either remove the line or comment it out to have all plugins re-enabled.