Timthumb.php Failed To Load

Hello,

I am running Wordpress 3.1 on Ubuntu. I am using the uBillboard photo slider plugin which uses timthumb.php to resize the photos. On occasion, the photo slider fails to load and hangs on the progress bar. (See screenshot 1)

The permissions are:
/wp-content/plugins/uBillboard/ - 755
/wp-content/plugins/uBillboard/timthumb.php - 644
/wp-content/plugins/uBillboard/cache - 777

I don't have any relevant errors in my apache error logs.

Safari's inspector shows Failed to load errors. (See Screenshot 2).

However, When I click on the timthumb.php link in Inspector, the links work fine. The links are:

http://domain.tdl/wp-content/plugins/uBillboard/timthumb.php?src=http%3A%2F%2Fdomain.tdl%2Ffiles%2F2011%2F02%2Fdefault-slide-4.jpg&w=661&h=271&zc=0&q=80

I am not sure why these are failing to load, when they work fine if I access the link directly. I suspect this has something to do with Wordpress's url rewriting?