When creating a folder it just hangs at "Creating folder..." eventually timing out. Firebug shows a 404 error and response "Invalid Request"

Keepalives on this page also have the same problem.

Looking at the request and the source of index.php it looks like the problem is the folder creation action isn't sending the X-Roundcube-Request header, causing an error from the CSRF prevention code.

How can I fix this so it includes the missing header?