I don't really know what is causing this but everything on the page seems ok (except for where the $OL line is... it should be right after the <body> tag).
For some reason, when clicking on a link to "calendar_in.php" your server redirects the request to /calendar_in.php instead of staying in the same folder. What you could do is you could move the file calendar_in.php to your /htdocs/ folder and adjust the paths in your requires to be "calendar/calendar.php".
Another possible solution would be to add the $thisFile argument right before the calendar and set it to "/calendar/calendar_in.php" and that should fix your links, but I'm not too sure on how your server will handle them.
Check to make sure you don't have some kind of URL Rewriting rule enabled.
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