filebrowser telling me Sorry, can't map to a physical path

The short story is, I'm getting the error "Sorry, can't map "UserFilesPath" to a physical path. You must set the "UserFilesAbsolutePath" value in "editor/filemanager/connectors/php/config.php". I get this error when I attempt to interact with the file browser (browsing, uploading, creating new folder, anything).

The longer story: I have this all working on my local webserver (XAMPP install). FCKeditor 2.6.6 is installed inside my project folder at "/sbexpress/fckeditor". I want to store all uploads that come in through the Filebrowser at "/sbexpress/pagefiles", putting images at "/sbexpress/pagefiles/images" and thumbnails at "/sbexpress/pagefiles/thumbnails". I have changed in the file "/sbexpress/fckeditor/editor/filemanager/connectors/php/config.php" the UserFilesPath to "/sbexpress/pagefiles/". I have left the UserFilesAbsolutePath blank.

One other thing to note, I am using CodeIgniter framework, which monkeys with URLs a bit.

This all WORKS when using a webserver (XAMPP) locally. However, as soon as I upload the whole ball of wax to my domain, so that the URL for my project becomes http://mywebsite.com/sbexpress, I get the "can't map to a physical path" error. Huh? I don't get it.

The kicker is, if I point my browser to "http://mysebsite.com/sbexpress/fckeditor/editor/filemanager/connectors/test.html", I can use with great success the upload form on the test page. I can perform the "Get Folders", the "Get Folders and Files", and the "Create Folder" commands, all with no problem! Here's the XML from the Get Folders command:

<Connector command="GetFolders" resourceType="Image"><CurrentFolder path="/" url="/sbexpress/pagefiles/images/"/><Folders><Folder name="Test Folder"/></Folders></Connector>