Generic Q and A: HTTP Error 503

Jaron sent me a message:

I'm trying to implement the SWFUpload Module onto a Drupal installation.
Some pertinent information:

Hosting: HostGator
PHP: 4.4.4
MySQL: 4.1.22
Drupal: 5.3 (Fantastico Installation from Hostgator...possibly relevant
because I had to manually install the update.php file...maybe there's
something else defective about Fantastico's version...)
SWFUpload: 5.x-1.1
SWFUpload Source Files: 1.0.2-min (from http://swfupload.mammon.se)

I fixed the Title Case Status Error using the line replacement covered in
the forums (SWFUpload.js / swfupload.js). Overall, the module is *very*
close to working. The flash component DOM replacement completed in my File
Attachment forms.

The specific problem I'm facing is that when file uploads complete in the
Flash component, the exact moment they complete, I get a JavaScript alert
box with the error:

"Error Code: HTTP Error, File name: 2.2 Anatomy Of Pelvis.pdf, Message:
503"

Observations:
- normal AJAX file uploads attachments work fine

What I Have Tried:
- chmod +777 on the files directory
- using SWFUpload: 5.x-2.x + SWFUpload v7.00beta3. With this combination,
I can't even get the Flash component to replace in the form, hence the
reason why I'm trying to make this work with the 5.x-1.1
- poking around trying to figure out what is feeding the swfupload.module
the HTTP 503 Error to no avail

Questions:
1) Are there any module dependencies that I might not be aware of?
2) What could the HTTP Error 503 mean?
3) Could my questions get any more generic? :D

Thank you so much for any assistance you could provide and for maintaining
such an awesome module. I've used SWFUpload on other PHP websites before
and when it does work, the thing flies like butter so I'm really looking
forward to getting it to work on my installation.